描述疏松砂岩油层时，若砂岩数量（）难以判断油砂百分比，可（）油砂的颗数。
参考答案：太少；单记
Many scientists believe that the universe began with an enormous explosion called the “Big Bang”, ______ happened about 13.7 billion years ago.
A．that
B．which
C．when
D．what
吃梅止渴是______的反射，望梅止渴和谈梅止渴都是______的反射．
但望梅止渴是人和动物______的反射，而谈梅止渴是对______产生的反射，人类______．
