电梯检验时发现绳槽磨损极为严重(铁屑较多)可能产生的原因是()。
参考答案:材质问题、张力不均、钢丝绳选用不当安装不当、比压过大
The computer center, ________ last year, is very popular among the students in this school.
A.open
B.opening
C.have opened
D.opened
数据流图的类型有______和事务型。