send sb to do sth意思是让某人去做某事.这里do sth.是目的.如:I sent my brother to get some milk.
send sb doing sth.意思是让某人开始做某事.这里sth.只是强调一个动作,并非目的,如:The punch sent him staggering towards the door.(这一推让他向门冲去.)
PS:1.D.be sent there to be trained for a space flight.送去那的目的是进行航天飞行训练,是目的,所以用send sb to do.
2.B.被一个长途电话叫去赶去见母亲,是强调hurry这个动作,而非目的,所以用send sb doing.
不知道清楚了没?