题目

Since English biologist Charles Darwin (1809 –1882) published On the Origin of Species in 1859, scientists have vastly improved their knowledge of natural history. However, a lot of information is still the subject of speculation, and scientists can still only make educated guesses at certain things.One subject that they guess about is why some 400 million years ago, animals in the sea developed limbs (肢) that allowed them to move onto and live on land.Recently, an idea that occurred to the US paleontologist (古生物学家) Alfred Romer a century ago became a hot topic once again.Romer t声音能靠                  传播，它是一种                   ，它能够粉碎石子，因而它还具有                   。声音在水、空气、铁棒中传播速度由快到慢的顺序排列是                               。月球上没有空气，登上月球的宇航员即使相距很近，也无法听到对方讲话的声音，这是由于                    不能传声。