Which of the following is a valid argument?

A$)$ All humans are mortal. Socrates is a human. Therefore, Socrates is mortal.

B$)$ All cats are mammals. Some mammals are dogs. Therefore, some dogs are cats.

C$)$ All flowers are plants. No plants are animals. Therefore, all flowers are animals.

D$)$ All birds can fly. Penguins are birds. Therefore, penguins can fly.