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.

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