many young people in the west are expected to leave what could be life's most important decision-marriage-almost entirely up to luck 中为什么用what不用that或which

问题描述:

many young people in the west are expected to leave what could be life's most important decision-marriage-almost entirely up to luck 中为什么用what不用that或which

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此句为定语从句,没有先行词,先用something代先行词,再加that.
something that=what

what could be life's most important decision what是这个从句的主语