阅读下面短文，从短文后所给各题的四个选项（A、B、C和D)中，选出可以填入空白处的最佳选项，并在答题卡上将该项涂黑。
People usually get into a hard life and are unable to escape from themselves.
There was once a poor and disappointed salesman complaining every day that there was no
for him to display his ability and the
was often pulling his legs.
At Christmas Eve, every family
their houses with lanterns and streamers and it was very
everywhere. But he was alone sitting in a bench in a park and recalling the past. It was on the same day last year when he was also alone
his Christmas day away,
new clothes, new shoes, let alone a new car and a new house.
“Damn! I have to spend this Christmas day with these old shoes again. ” he
and began to take off the old shoes.
, he glanced at a young man in a wheel chair passing by him with his hands strugglingly pushing the wheel forward. It
him that he was so lucky to have shoes to wear while that man did not even have the
. Afterwards, the sales man did anything with a calm
and cherished every opportunity to improve himself. He worked hard and tried his best to make progress every day. Several years later, he
changed his life totally and became a millionaire.
If you look all around, you will find there are so many people who were born
in the society but they are
in life and never complain about the unfair destiny. They are also not to beg others’ giving. Instead, they constantly
to make themselves stronger and more excellent to
the society. By contrast, we should feel
. We are born healthy, but we are
with life; we complain about our colleagues and dissatisfy with our jobs.
Maybe all of us would feel afraid when we realize we have fallen into such
state of life, but worse is that you even do not
you have fallen into such a dangerous situation.
质量为M的长木板放在光滑水平面上，一个质量为m的滑块以速度v沿木板表面从A点滑到B点，在木板上前进了L，而木板在水平面上前进了s，如图所示．设滑块与木板间的动摩擦因数为μ．求：