低位性肠梗阻呕吐物呈A．腐臭味B．酸臭味C．粪臭味D．腥臭味E．腐饭味

I seemed to be pretty good___ reading his lips after a while, even better than the nurses that were caring ___ him.

A．at; for

B．for; for

C．at; about

D．for; about

有三个共点力F₁=2N，F₂=4N，F₃=4N，互成120°角，其合力的大小为______N．

常用防止食品腐败变质的措施有（）、（）、（）、（）和（）等。