如图5-48所示，梁为木梁时最大正应力为σ_a，把该梁换成低碳钢梁其他条件不变时最大正应力为σ_b，则σ_a与σ_b的关系为（）。

A．σ_a＞σ_b

B．σ_a＜σ_b

C．σ_a=σ_b

D．无法比较

Many famous athletes do a lot of work to help people who are not as              as themselves.[ ]

A. lucky

B. unlucky

C. luckily

D. unluckily