when he wakes up,he-----out of bed at once

when he wakes up,he_____out of bed at once
A.is getting
B.got
C.gets
D,was getting
这里是选C麼,我就是搞不清楚,出现at once这个单词是跟动词原型还是ING
求达人们为小弟解释解释,感激不尽
2L..
你又说过去式，怎麼又用原型?