Exercise in propositional logic. Which of the following arguments is valid? 
A. If it rains, then the grass grows. The worms are not happy unless it rains. Therefore, If the worms are happy , then the grass grows.
B. If the wind howls, then the wolf howls. If the wind howls, then the birds sing. Therefore, if the birds sing, then the wolf howls.
C. If the sun shines, then it is day. If the stars shine, then the sun does not shine. Therefore, if the stars shine, it is not day
D. Both A and C.

If D) is valid then A) and C) are also valid; therefore there will be three valid statements.
If your don't select one of A),B) or D) then your answer is wrong.
There is only one true argument, and is not B).
