____提出了自然选择学说,并著有《物种起源》.
现有CH4和C2H6(乙烷)组成的可燃性混合气体,该混合气体中氢元素的质量分数可能是( )
— There is somebody under the tree. Is it Mr. Lee?
— It ________ be Mr. Lee. He left for Beijing this morning.