At night or during poor visibility by day, lights are _______ . For lights to be effective, they must be of adequate intensity.
A.ready
B.required
C.provided
D.served
正确答案是B
At night or during poor visibility by day, lights are _______ . For lights to be effective, they must be of adequate intensity.
A.ready
B.required
C.provided
D.served
正确答案是B