经口摄取半数致死量LD50≤（）mg/kg的固体为毒害品。
A、600
B、650
C、500
D、550
参考答案：C
In April, 2009, President Hu inspected the warships in Qingdao, ________ the 60th anniversary of the founding of the PLA Navy.
A．marking
B．marked
C．having marked
D．being marked
什么是离心泵的汽蚀？汽蚀是怎样产生的？
