A large number of engineers ___ to Africa by our government to help the people there every year.[ ]

A. have sent        

B. will send          

C. are sent        

D. was sent

下列说法正确的是[ ]

A、浓溶液一定比稀溶液的质量分数大

B、不饱和溶液转化为饱和溶液后，质量分数一定增大

C、同种物质的饱和溶液一定比它的不饱和溶液要浓

D、在一定温度下，同种物质的饱和溶液总要比其不饱和溶液要浓