介词之后的宾语从句,不可用which或if连接,要分别用what或 whether,为何?

因为which有选择的意思,是“哪一个/哪一些”的意思,并且不能在宾语从句中做单独作主语,所以换用what是“所...的事物”,可以在从句中做主语,例如：Something about what had happened did not seem right to the junior traffic department employee.
因为if不但有“是否”的意思,还有“如果”的意思,不如用whether这个只有“是否”一个词义的连词清晰、准确.例如：Jeff Sessions of Alabama,raised questions about whether it achieves necessary spending cuts and raises taxes