[单选题]为了保证行车安全，必须使信号机、（ ）和道岔三者之间，建立一种相互制约关系，这种关系就是联锁关系。
A.信号机
B.继电器
C.转辙机
D.进路

Generally speaking, a British is widely regarded as a quiet, shy and conservative person who is (1) only among those with whom he is acquainted. When a stranger is at present, he often seems nervous, (2) embarrassed. You have to take a commuter train any morning or evening to (3) the truth of this. Serious-looking businessmen and women sit reading their newspapers or dozing in a corner; hardly anybody talks, since to do so would be considered quite offensive. (4) , there is an unwritten but clearly understood code of behavior which, (5) broken, makes the offender immediately the object of (6) .
It has been known as a fact that a British has a (7) for the discussion of their weather and that, if given a chance, he will talk about it (8) . Some people argue that it is because the British weather seldom (9) forecast and hence becomes a source of interest and (10) to everyone. This may be so. (11) A. unless
B. once
C. while
D. as