They tried several times to       the problem, and in the end they succeeded.

A．give up

B．work out

C．look through

D．think of

遇到武装冲突，如必须移动，采取匍匐前进或者打滚方式，尽可能地保持低位，贴近地面并使用可以一切可利用的遮蔽物。