The whole class _____the teacher attentively.
A. are listening to
B. is listening to
C. are listening
D. has listened
查看答案
This is an illness that can result in total blindness if _____untreated.
A. left
B. being left
C. is left
D. it left
A reagent is any chemical that reacts in a predictable way _______withother chemicals.
A. when mixed
B. when is mixed
C. it mixed
D. when mixing
As a general rule, snakes don't bite unless ________.
A. are offended
B. they offended
C. offending
D. offended
Not until ________himself seriously ill.
A. he had completed the task did he find
B. had he completed the task did he find
C. he completed the task he had found
D. did he complete the task he had found
微信支付 
支付宝支付