标准煤是能源的统一计量单位。凡能产生（）低位热量的（）均可折算1Kg煤当量值。
参考答案：29.27MJ、任何能源
受精卵分裂成为16个细胞组成的桑葚胚约需几天（）
A.1
B.2
C.3
D.4
E.6
I had been puzzled over the problem for over an hour without any result, ____ all at once the solution flashed across my mind.
A．while
B．when
C．then
D．as
