请教一道英语时态题.14) He ____ for three years.A) has joined the arm

请教一道英语时态题.
14) He ____ for three years.
A) has joined the arm B) has been in the army
C) has been serving in the army D) joined the army
这道题做的时候我觉得B,C都可以.正确答案是B,为啥不能选C?
思路：join不能个能持续性的状态,我就去除了A
For 3 year需要搭配完成时,我就去除了D.
剩下B没有任何为题,C是现在完成进行时也表持续,为啥不对?
不好意思,手头没分了~好心人顺手一答,算是做善事吧~