没看懂句子的意思In the smoke and dust and darkness,people ____ to on

没看懂句子的意思
In the smoke and dust and darkness,people ____ to one another grabing hands and linking arms,they formed a human chain that made their way down stairwells and corridors and out into the sunlight of an unthinkable day
A reaching up B reaching down C reaching for D reaching out答案是选d……能不能帮我看一下句子什么意思再说明一下为什么选择d