完形填空。
[2014·吉林省长春市高中毕业班调研考试]
One day in winter, a couple of good friends headed for a frozen lake nearby to skate on ice. The boys were between five and ten years old. When they were skating, one of the boys headed deeper into the lake, but __1__ he found himself skating on very __2__ ice. Before he could __3__ it, he had fallen through the ice into the icy water below.
