单选题 Too much salt is known to contribute to hypertension, ______ is a factor in half the deaths in the U. S. each year.
A. what B. that C. which D. this

【正确答案】 C

【答案解析】摄入太多盐被认为会导致高血压；每年美国的死亡人数中一半都起因于高血压。
[答题点拨] 由横线前的逗号可知此处为which引导的非限制性定语从句。