[单选题]The paragraph is followed by an incomplete phrase or sentence which summarizes the main idea of the paragraph. Choose the missing words of the phrase or sentence. Happiness research suggests that neither very good events nor very bad events seem to change people's happiness much in the long term. Most people, it seems, revert back to some kind of baseline happiness level within a couple of years of even the most devastating events, like the death of a spouse or loss of limbs.

For the majority, there seems to be a （）for happiness level.