By the time he leaves university, he ______ some working experience from his part-time job.
A：will be gaining
B：will gain
C：has gained
D：will have gained
射箭运动员要把箭射出去，先把弓拉弯，这说明弹力是由于物体发生____而产生，要射中靶心，箭射出的箭能够继续运动，这是因为箭具有____。
