【单选题】【消耗次数：1】
He ___________ his friends for their help at the hotel now.
①
entertained
②
was entertaining
③
is entertained
④
is entertaining
参考答案：
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相关题目
【单选题】
The students were all entertained in a Mexican restaurant, at Professor Brians ____.
①
money
②
pay
③
expense
④
loss
【单选题】
If it _____ for his help, I wouldn’t be a university student now.
①
hasn’t been
②
hadn’t been
③
wasn’t
④
weren’t
【单选题】
In the past he often made his sister___, but now he is often made __by his sister.
①
to cry; to cry
②
cry
③
to cry; cry
④
cry; to cry
【单选题】
No one, except his two best friends ____his joy or sorrow in his life.
①
shares
②
share
③
sharing
④
to share
【单选题】
-Johnson has promised us that he will help ?us out to his best ability.
-I don’t buy it. In my opinion, ?he _______ so kind.
①
has been
②
is
③
was
④
is being
随机题目
【单选题】
关于奈氏判据及其辅助函数F(s)=1+G(s)H(s)，错误的说法是（）
①
F(s)的零点就是开环传递函数的极点
②
F(s)的极点就是开环传递函数的极点
③
F(s)的零点数与极点数相同
④
F(s)的零点就是闭环传递函数的极点
【单选题】
关于系统零极点位置对系统性能的影响，下列观点中正确的是（）
①
如果闭环极点全部位于S左半平面，则系统一定是稳定的。稳定性与闭环零点位置无关；
②
如果闭环系统无零点，且闭环极点均为负实数极点，则时间响应一定是衰减振荡的；
③
超调量仅取决于闭环复数主导极点的衰减率，与其它零极点位置无关；
④
如果系统有开环极点处于S右半平面，则系统不稳定。
【单选题】
关于线性系统稳态误差，正确的说法是：（）
①
一型系统在跟踪斜坡输入信号时无误差；
②
稳态误差计算的通用公式是；
③
增大系统开环增益K可以减小稳态误差；
④
增加积分环节可以消除稳态误差，而且不会影响系统稳定性。