皮带输送机的主要作用就是靠它的皮带将工件输送到下一工序，通过（）或速比控调节到所需速度进行工作的。
参考答案：变频器
Since she is out of work, Lucy________ going back to school, but she hasn’t decided yet.
A．had considered
B．has been considering
C．considered
D．is going to consider
设矩阵A=(aij)4×4，B=(bij)4×4，且aij=-2bij，则行列式|B|=
A．2-4|A|．
B．24|A|．
C．-2-4|A|．
D．-24|A|．
