使用含磷洗涤剂(含有Na5P3O10)会引起水域含磷量升高,造成环境污染,Na5P3O10中磷元素的化合价是( )
A:﹣3
B:+2
C:+3
D:+5
He____(go) to school on foot many years ago. Now, he____(go) to school by bus.