--It’s a pity that you didn’t come to my birthday party last night.

--I’m terribly sorry. I ________ for an important lecture.

A．prepared

B．was preparing

C．had prepared

D．has prepared

如图所示每条折线表示周期表ⅣA～ⅦA中的某一族元素氢化物的沸点变化，每个小黑点代表一种氢化物，其中a点代表的是(　　)

A．H₂S

B．HCl

C．PH₃

D．SiH₄