计算机开机后，键盘指示灯不亮，则可能是（）。
A、外壳老化
B、弹簧变形
C、接口松脱
D、电极积垢
参考答案：C
请你用所给词的适当形式完成下列句子。
1. Here are some things they have learned from ______(scientist ) studies.
2. Sunglasses are used for _____ (keep) out the sun.
3. The students are ______ (confuse) about the ______ (confuse) problem.
4. I’m considering _____ (study) in Beijing or Shanghai.
5. I felt _______(embarrass) when everyone looked at me.
双线性变换法是非线性变换，所以用它设计IIR滤波器不能克服频率混叠效应。（）
