Question

These diagrams are especially helpful when we have a concrete problem ________ at hand.

A．be solved	B．to solve
C．solved	D．being solved

Answer 1

B

B 考察非谓语动词。句义：如果我们手边没有解决的具体问题，你们这些数据就非常有用，have在此表示有，后面接不定式的形式。