Up to now, large quantities of food         to Africa to save the starving people.

A．has been sent

B．have been sent

C．are sent

D．is sent

电泳法分离蛋白质时，缓冲液的离子强度的一般要求是（）。

A.0.01～0.05

B.0.02～0.1

C.0.02～0.2

D.0.1～0.2

E.0.2～0.5