There are still many problems _______before we are ready for a long stay on the Moon.
A．solving
B．solved
C．being solved
D．to be solved

参考答案：D 答案解析： 句子主干在there are still many problems，后面部分是用于修饰problems的定语，这里选择的是非谓语动词的正确形式，问题是还没被解决将来将要被解决，所以用不定式表示将要发生，故选择D。