弹簧式缓冲器试验时，配合人员将载有额定载荷的轿厢以检修速度撞击缓冲器，直至曳引绳在曳引绳槽上打滑。( )
参考答案：对
______ he became seriously ill, he still went on working.
A．If
B．Unless
C．And
D．Even though
什么叫导线初伸长？
