问题
Do you know about a very popular toy— the yo-yo(悠悠球)—Yes. Filipinos(菲律宾人) invented it ________ accident. And it ________ helping kill animals.
A:at; used to
B:on; was used to
C:by; used for
D:by; was used for;
B:on; was used to
C:by; used for
D:by; was used for;
正确答案是：
