flag是什么意思

When you forget entire experiences, he says, that's "a red flag that something more serious may be involved."

2018年6月四级真题（第二套）阅读 Section C

More than a century has passed since explorers raced to plant their flags at the bottom of the world, and for decades to come this continent is supposed to be protected as a scientific preserve, shielded from intrusions like military activities and mining.

出自-2016年12月阅读原文

Because their behaviors are not usually seen as a red flag, these young people have been dubbed the invisible risk group by the study's authors.

出自-2016年12月阅读原文