核能的释放,通常有两种形式,一种是重核的(),另一种是轻核的聚变。
参考答案:裂变
When I dropped in, his family _______at the table, talking about something.
A.sat
B.was sitting
C.have sat
D.were sitting
等差数列an的前n项和为Sn,若S2=4,S4=20,则该数列的首项为(),公差为()。