How many cities has the Smiths traveled to since they came to China last week?

A.have,traveled B.was,traveled
C.did,travel to D.has,traveled to
麻烦各位这道题怎么做，并解释一下各个选项为什么合理和不合理的原因，好吗？谢谢了！

How many cities have you travelled to ?中"to"可以省略吗?为什么? How many cities have you been to in China?的意思是什么？该怎样回答？ How many cities has the Smiths traveled to since they came to China last week? How many how many How many的问题 How Many Fools How Many Elements Now many cities in China ____ water pollution 多少杯水用how many 还是how much