A. What was not realized in those early years was that birds have muscles very much stronger, in proportion to their size, than men.B. Experiments were carried out in many countries, sometimes with models driven by steam engines, but these were too heavy to be used in an airplane with a pilot.C. Professor Smith has been invited to give us a lecture on how to choose our favorite career.D. When you go out shopping and your child asks for something, buy a book. They’re cheaper than toys and a far better investment in your child’s future.

先前人们没有意识到的是：相对于自身的体积来说，鸟类拥有比人类强壮得多的肌肉。

A．
B．
C．
D．