SAT二道语法题,1.After [many years] of training,some artists are so skilled [in reacting at] any stimulus that they [can quickly] retaliate if attacked.[in reacting at] 错,求这个词组的正确用法/固定搭配,2.A a person [need not] purchase tools.(我简化成了题干)这里[need not]没错,请问为什么不是"needs"

问题描述:

SAT二道语法题,
1.After [many years] of training,some artists are so skilled [in reacting at] any stimulus that they [can quickly] retaliate if attacked.
[in reacting at] 错,求这个词组的正确用法/固定搭配,
2.A a person [need not] purchase tools.(我简化成了题干)
这里[need not]没错,请问为什么不是"needs"