When something is on fire, people throw sand on it to put out the fire. The main use of the sand is to _________.

A．protect the thing on fire

B．add weigh to the thing on fire

C．keep the air away from the thing on fire

D．lower the temperature of the thing on fire

混合淋巴细胞反应可用于（）。

A.体外评价B淋巴细胞的功能

B.筛选骨髓移植的供体

C.体内评价T淋巴细胞的功能

D.体外评价T淋巴细胞的功能

E.测定外周血T淋巴细胞的数目