单选题
A. unnecessary
B. complete
C. lightest
D. important
E. flammable
F. reactive
单选题
Noble gases are not very chemically______.
【正确答案】
F
【答案解析】【解析】题意：惰性气体化学性质不……。根据第三段第二句These properties include being monatomic，colorless，odorless，being able to conduct electricity，and having low chemical reactivity可知惰性气体化学性质不活泼。故F选项“活跃的”合适。
单选题
Among the elements of noble gases Helium is the____.
【正确答案】
C
【答案解析】【解析】题意：在惰性气体中，氦是……。
根据文中的第三段最后一句Helium has the lowest molecular weight while Radon is the heaviest可得知，氦气是所有惰性气体中最轻的。故选C。
单选题
The required number of electrons in noble gases` outer shell is _____.
【正确答案】
B
【答案解析】【解析】题意：惰性气体外层电子数应为……。根据第四段第三句Noble gases have a full outer shell，meaning that they have complete electrons in their outer shell可得知，惰性气体外层电子数应全满。故选B。
单选题
MRI may make operating on the patient ______.
【正确答案】
A
【答案解析】【解析】题意：磁共振成像(MRI)可使手术……。根据第五段最后一句MRI allows the doc— tot t0“see”the brain，without operating on the patient可知，通过MRI，医生可以不通过手术就看到患者头部影像。故选A。