杂交之前，骨髓瘤细胞必须缺陷()

A．叶酸

B．HGPRT

C．氨基蝶呤

D．胸腺嘧啶核苷

E．次黄嘌呤

Racing the clock every day is such an exhausting effort that when I actually have a few free moments, I tend to collapse.

A．fall down

B．fall ill

C．fall out

D．fall behind