根据下列内容，回答216-220题。
The men who race the cars are generally small, with a tight, nervous look. They range fromthe early 20s to the middle 40s, and it is usually their nerves that go first.
Fear is the driver's constant companion, and tragedy can be just a step behind. Scarcely a manin the 500 does not carry the scars of ancient crashes. The mark of the plastic surgeon is every-where, and burned skin is common. Sometimes a driver's scars are invisible, part of his heritage. Twoyoung drivers, Billy Vukovich and Gary Bettenhausen, raced in their first 500 in 1968. Less than 20years before, their fathers also competed against one another on the Indy track--and died there.
All this the drivers accept. Over the years, they have learned to trust their own techniques,reflexes, and courage. They depend, too, on a trusted servant--scientific engineering. Though theymay not have had a great deal of schooling(an exception is New Zealand's Bruce McLaren, whohas an engineering degree), many drivers are gifted mechanics, with a feeling for their enginesthat amount to kinship.
A few top drivers have become extremely wealthy, with six-figure incomes from prize money,endorsements, and jobs with auto-product manufacturers. Some have businesses of their own.
McLaren designs racing chassis (底盘). Dan Gurney's California factory manufactured the chassisof three of the first four ears in the 1968 Indy 500, including his own second place car.
Yet money is not the only reason why men race cars. Perhaps it isn't even the major reason.
Three-time Indy winner (1961,1964,1967) A. J. Foyt, for example, can frequently be foundcompeting on dirty tracks in minor-league races, where money, crowds and safety features are limit-ed, and only the danger is not. Why does he do it7Sometimes Foyt answers,"It's in my blood."
Other times he says, "It is good practice." Now and then he replies, "Don't ask dumb questions."
The statement "it is usually their nerves that go first" means

A.at first they all have a nervous look
B.they often find they can't bear the tension even if they are in good condition
C.someday they find they can't make responses to any risk
D.they can continue their career at most until the middle 40s