选择填空。

— Are those your tennis rackets?

— No, ______. [ ]

A. it isn't

B. they are

C. those aren't

D. they aren't

真空系统气密包括（）。

A、UFPP及其EG喷淋冷却系统

B、FIN及其EG喷淋冷却系统

C、酯化系统

D、四级真空喷射泵系统