What caused the tragedy to happen according to the authorities?
A： Overcrowding and the failure of the police to contro1 the crowds.
B：Unsafe exits and small squares to hold enough people.
C：Bad weather and dark lights to guide people to leave.
选择客运或货运方式需要考虑的因素，与____ 的高低、____ 的快慢、____ 的多少有很大关系。
