a). nest b). tree c). gullet d). crop

Answer the following question given this paragraph: Birds have a sac-like structure called a crop to store and moisten food that is waiting to be digested. They also have an organ called a gizzard that contains swallowed stones. The stones make up for the lack of teeth by grinding food, which can then be digested more quickly. Both structures make it easier for the digestive system to produce a steady supply of nutrients from food. Q: Where do birds store and moisten food that is waiting to be digested? A:
Answer:
d).