There are some police cars in front. What do you suppose ____？ 

A. is the matter 

B.is happened 

C.is the wrong 

D.the trouble is
请问这道题为什么不能选B或D？

   【答】happen是不及物动词，没有被动语态形式，所以 B.is happened 是错误的。  wrong  是形容词，所以 C.is the wrong 是错误的。可以说 What's (What is) the trouble ? ，这个句子的主语是 What，如果把 What 移到 do you suppose 前面，其余部分应该是 is the trouble， 不是 the trouble is，所以不能选D。

    What's (What is) the matter? 

   = What's (What is) the trouble ? 

   = What's (What is) wrong? 

   who/what/where etc do you suppose? spoken 【口】 used for asking someone what they think about something  您认为谁 .../ 什么 .../ 在哪里...? 等：Why do you suppose he's so angry? 你认为他为何如此生气？What do you suppose will happen now? 你认为现在会出现什么情况呢？（摘自《麦克米伦高阶英汉双解词典》p.2112）