单项选择题
A. the nucleus of a cell B. cloned human beings C. a human being in two years D. a law to ban human cloning E. a report on human cloning F. heavy media coverage
A．They were formed in 1984． B．They are at the top of two active volcanoes． C．They are not like most other crater lakes． D．Water in them turns over regularly．
A．could help store messages． B．could send messages instantly C．could tell when it is wise to interrupt you． D．could identify important phone calls．