液下喷射氟蛋白泡沫，喷入储罐内的速度越快，泡沫中的含油量就越多。因此，为保证泡沫的灭火效能，泡沫喷射的流速不应大于（）

A、1m/s

B、2m/s

C、3m/s

D、4m/s

—Why didn’t David ________his friend’s invitation to the picnic?

—Because he had to prepare for the exam.

A．accept

B．receive

C．refuse

D．get