如果某三相异步电动机的极数为4级，同步转速为1800转/分，那么所接三相电源的频率为（）。 ["A、50赫兹 ","B、45赫兹 ","C、60赫兹 ","D、30赫兹"]
Which clause should you use to exclude group results? （） [" WHERE"," HAVING"," RESTRICT"," GROUP BY"," ORDER BY"]
持球突破技术主要是由（）、（）、（）、（）、（）五个技术环节组成。持球突破技术动作是由（跨）、（）、（）、（）四个环节组成。
下列不属于隐性广告表现形式的是（） ["网络新闻广告","BBS广告","搜索广告","全屏广告"]
Why would you use the following FLASHBACK TABLE command？（） FLASHBACK TABLE emp TO TIMESTAMP （’11：45’，’hh12：mi’）； ["to undo the changes made to the EMP table since the specified time","to restore the EMP table that was wrongly dropped from the database","to view the transactions that have modified the EMP table since the specified time","to view the changes made to the EMP table for one or more rows since the specified time","to recover the EMP table to a point in time in the past by restoring the most recent backu"]
简述微生物的主要特征。
