18. Both Serena，who has just been(A) elected president of the union，and(B) Martin，its treasurer，attends(C) the weekly meetings(D) of the labor council. No error(E)

　　答案：C

　　类型：主谓一致

　　解析：句子的主语是both...and...，因此谓语动词不能用三人称单数。

　　修改建议：把attends改为attend。

　　19. Although(A) artificially intelligent machines can now(B) beat humans at chess，they cannot outperform(C) humans at practical tasks such as(D) folding laundry，cooking，and caring for perts. No error(E)

　　答案：E

　　类型：no error

　　20. When one thinks about opening(A) a new business，you(B) must consider(C) factors such as(D) start-up costs，local competition，and customer demand. No error(E)

　　答案：B

　　类型：代词

　　解析：one是泛指，其对应的特指应该是第三人称he or she。

　　修改建议：把you改为he or she。

　　21. Automatic photo booths that were(A) invented after the discovery of an automated photographic process that eliminated(B) the need(C) not only for negatives but also for(D) camera operators. No error(E)

　　答案：A

　　类型：句子结构

　　解析：句中两个动词were和eliminated都是从句中的动词，所以句子主句没有谓语动词。

　　修改建议：把that删掉。

　　22. Faced with(A) the ever-challenging task of balancing(B) school and work，Jillian opted to(C) forego taking a second part-time job and focusing(D) on her studies instead. No error(E)

　　答案：D

　　类型：平行结构

　　解析：从语义来进行分析：focus应该和forego是平行的，而不是和take平行。所以不能用doing形式。

　　修改建议：把focusing改为to focus。