题目选项分析

Avalanches at Rogers Pass in Glacier National Park killed more than 200 people between 1885 and 1910, but such avalanchesare now controlled if not prevented; cannons are fired at the slopes to make snow masses fall before they become dangerous.

More than 200 people have been killed by avalanches between 1885 and 1910 at Rogers Pass in Glacier National Park, but theyare now controlled if not prevented; cannons are fired at the slopes to make snow masses fall before they become dangerous.