2.The center of gravity of the human body ______ behind his joint.
A. located B. locating C. to locate D. is located
5.“When did you go to work?”
“As soon as they came, we ______ to work.”
A.went B. were going C. had gone D. go
7.Perhaps it will be A long time ______ from abroad.
A. when Tom comes back B. when Tom will come back
C.before Tom comes back D. that Tom comes back
8.Tom ______ mathematics throughout his college life.
A. bored B. bored with C. was bored D. was bored with
12.My English teacher ______ my test.
A.has not yet graded B. has not yet been graded
C.is not yet to be graded D. is not yet graded
13.We ______ on it for many hours, but we have not yet reached any conclusion.
A.are being worked B.are working C.have been working D.have been worked
16.“______ your best? It’s not satisfactory. I’m afraid you will have to do it again.”
A.Do you try B. Have you tried C. Are you trying D. Have you been trying
18.My uncle, Sam,______ manager of the firm.
A.has just made B.is just being made C.has just been made D.is just made
19.This is the first time the students ______ to Hyde Park.
A. have gone B. have been gone C. have been going D. are being gone
20.She ______ her mother’s work since she was admitted into hospital.
A. is doing B. has been doing C. has done D. has been done
详细解释一下原因，我知道挺多的，好的我追加！
第二题什么意思？
第十三题为什么不是D？
就这两个问题，非常谢谢你！
第二题：人体的重心在他的关节后面。 字面意思如此，具体可能还得结合上下文。不太通。但是语法是没有问题的。
第十三题：选答案D的话，变成现在完成时的被动语态了，have + been + 过去分词，我们被工作吗？肯定不对啊。如果用worked，直接用完成时态就好了——We have worked on it . （不用加been)，但是后半句“but we have not yet reached any conclusion”提示这个动作在说话之前仍在进行，故用现在完成“进行”时态。如果四个选项中有一个答案是“have worked"的确会造成困扰（因为上半句没有语法错误），好在此题目还比较人性，故意在答案D里放个"been"在那里，所以答案D连想都不用想就知道是错的。