问题
选择题
___ is the most useful invention?
A．Do you think which of these
B．Which of these you know
C．Which of these do you know
D．You know which of these
答案
答案：C
考察句型，do you know是插入语，与句子结构无关，放在句中。
句意为，你知道这些发明里面哪个是最有用的发明？