doctor | dog(s)
e.g., Do you have a dog, doctor?/The doctor said that he had a dog./My mother is a doctor and she loves dogs.
class | food
e.g., The class wants to have some food./Is food allowed in the class?/The teacher told me to bring the food to the class.
clean | cat(s)
e.g., Cats clean themselves very much better than any other pets./ Immediately clean up the mess that your cat just made, Johnny.