So convenient ______ to pay by credit card that more and more people apply for their credit cards. [ ]

A. do people find

B. people find it    

C. find people  

D. do people find it

如图表示人体的特异性免疫过程，请据图判断下列说法正确的是（）

A.经过⑤免疫可以将结核杆菌完全消灭

B.HIV侵入人体对⑤⑥，尤其对⑤有破坏作用

C.能特异性识别抗原的细胞有a、d、f

D.所有抗原侵入人体都需要⑤⑥共同作用