---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.
I won’t leave here until my mother _______ back.
We really don’t know _______. Could you help us?
—Will you please ________ do that? —OK, I won’t.
We had fun ________ the robots do many different kinds of things.
The boss makes the workers ________ long hours every day.