the professor left not much money but a lot of books __when he died.这个空里应该填什么介词,为什么补充下，空里可以填的是介词或副词。
问题描述：
the professor left not much money but a lot of books __when he died.这个空里应该填什么介词,为什么
补充下，空里可以填的是介词或副词。
答