—We can        a bus to Shanghai. Bus tickets(票) are cheap.

— I want to go there        train. It has a lot of fun.

A．take; on

B．by; by

C．take; take

D．take; by

相同的装填条件下，三叶草型催化剂床层的空隙率（）球型催化剂床层的空隙率。

A、小于

B、大于

C、等于

D、不能确定