Both of his parents died of illness so he ______ support his family at the age of 14.[ ]

A. must

B. should

C. could

D. had to

YB65型条盒包装机出现胶水缸锁不紧的原因是条盒胶水缸不到位、胶水缸锁紧手（）柄和胶水缸位置检测器失效。