7. Tom keeps 13 snakes, 4 of which are purple, and 5 are happy. Tom has found some interesting phenomena: all happy snakes can do addition, all purple snakes cannot do subtraction, and all snakes that cannot do subtraction also cannot do addition. Which of the following statements is correct? ( ).
(A) Purple snakes can do addition
(B) Purple snakes are all happy snakes
(C) Snakes that can do addition are all purple snakes
(D) Happy snakes are not purple snakes
(E) Happy snakes cannot do subtraction
7. D.

All purple snakes cannot do subtraction, snakes that cannot do subtraction also cannot do addition, all happy snakes can do addition, so snakes that cannot do addition are not happy. Therefore, purple snakes are not happy. Thus, happy snakes are not purple snakes.