My brother is going _________ to be a doctor.
A：study medicine
B：to study medicines
C：to study medicine
D：studying medicine
He never __________ to help those in trouble because he was a kind-hearted man．( )
我国人口最多的少数民族是（ ）
基因型为EeXBXb的个体，产生的配子名称和基因型种类是 ( )
两台机器的功率之比为5:4，若它们做的功相等，则所用时间之比为（ ）
