请高手指点

No senator spoke at the convention unless he or she was a Democrat. No Democrat both spoke at the connvention and was a senator.

Which one of the following conclusions can be correctly drawn from the statement above?

(A) No one but senators spoke at the convention.

(B) No Democrat spoke at the convention.

(C) Only Democrats spoke at the convention.

(D) No senator spoke at the convention.

(E) Some Democrat senators spoke at the convention.

答案为A, 不明白为何？请指点。