—Which flight ________ ? —I want to catch the 13:00.

A．you are to catch

B．are you catching

C．do you catch

D．have you caught

溶菌酶是一种黏多糖水解酶，能溶解细菌的（）

A.细胞核

B.细胞蛋白

C.细胞壁

D.DNA

E.RNA