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.

许多国家做了实验，有时用的是蒸汽机驱动的模型，但是这些模型对于由一个飞行员操作的飞机来说都太重。

A．
B．
C．
D．