
Chain Reaction III: The third annual "Chain Reaction" report grades America's 25 largest fast food and "fast casual" restaurants on their antibiotics policies and meat sourcing practices. These eleven restaurant chains received an F grade for the third consecutive year due to lack of a meaningful antibiotics policy. Nine of these companies didn't respond to the survey at all.

Chipotle - Grade: A "We started on our quest to serve meat from animals raised without the use of antibiotics in 1999, and continue to be a leader in this area today," Chipotle spokesman Chris Arnold said in an email. "We made the decision to serve antibiotic-free meat simply because we thought it was the right thing to do -- the right thing for farmers, for animal welfare, and for human health -- and we continue to hold the belief today."

Panera Bread - Grade: A "We're proud to have led the way on antibiotic reduction for more than a decade, starting with introducing chicken raised without antibiotics in 2004," Sara Burnett, director of wellness and food policy at Panera, said in a statement. "Today at Panera, 100% of poultry, bacon, breakfast sausage and ham served on sandwiches and salads is raised without antibiotics."

Subway - Grade: B+ "Our U.S. restaurants only serve chicken raised without antibiotics and we continue to work towards the elimination of antibiotic use in our Turkey, Pork and Beef supply," Subway said in a statement. "Antibiotics are critical tools for keeping animals healthy and should be used responsibly to preserve their effectiveness in veterinary and human medicine. We are proud of our progress and are committed to continued work in this space."

Chick-fil-A - Grade: B "Our efforts in the journey toward serving only chicken raised with No Antibiotics Ever (are) something our customers told us they wanted," Rob Dugas, vice president of Chick-fil-A's supply chain, said in a statement. "We have converted more than 50% of our supply to chicken raised with No Antibiotics Ever to date, and we are on track to achieve our goal of converting 100% of our chicken supply by December 31, 2019."

Taco Bell - Grade: B- Taco Bell did not immediately respond to CNN's request for comment.

KFC - Grade: B- "We're proud to be the first major chicken chain to commit to removing antibiotics important to human medicine in all of our chicken, including bone-in chicken," the company said in a statement to CNN. "This change presented a unique challenge for us because we serve chicken-on-the-bone. As such, this move required close collaboration with our suppliers who bring KFC chicken from the farm to our kitchens where it is hand-prepared."

McDonald's - Grade: C+ "Moving to cage-free eggs by 2025, sourcing chicken raised without antibiotics important to human medicine since 2015, and introducing McNuggets with no artificial flavors, colors or preservatives are all examples of our continued commitment to our US customers," Marion Gross, senior vice president for McDonald's North America supply chain, said in a statement. "Because we know we can and must do more across the globe, last month we also announced that in 2018 we will begin implementing a chicken antibiotics policy in markets around the world. And we remain committed to making meaningful reductions in the use of antibiotics in beef and pork and will share our progress on beef in 2018."

Wendy's - Grade: C "We continue to build on the progress we've made in phasing out the use of medically important antibiotics in our supply chain where possible," the company said in a statement. "Wendy's is committed to accomplishing this with the help of suppliers, farmers and ranchers. We believe the steps we are taking will make a difference in protecting the future health of people and animals."

Pizza Hut - Grade: D+ Pizza Hut did not immediately respond to CNN's request for comment.

Starbucks - Grade: D+ Starbucks did not immediately respond to CNN's request for comment.

Dunkin' Donuts - Grade: D "We do not own, raise, process or transport livestock, but we recognize that animal welfare is an important part of a safe and sustainable food supply chain," the company said in a statement. "By the end of 2018, any chicken offered in Dunkin' Donuts restaurants will be sourced from chickens raised with no antibiotics ever."

Burger King - Grade: D Burger King did not immediately respond to CNN's request for comment.

Papa John's - Grade: D Papa John's did not immediately respond to CNN's request for comment.

Jack in the Box - Grade: D Jack in the Box did not immediately respond to CNN's request for comment.