—Is there any vaccine(疫苗) against H1N1 flu that can be used on humans?—No, but scientists are working to develop ________ .
They dug _____,then put the box in it.
Glad to see you back. How long __ in Russia?
They lived a ____ life in the countryside.
---Have you decided already? ---Yes, I __ at once.
His success is____ by talent as by energy.