40. ______ trees are planted each year on the Tree-Planting Day all over the country.

　A. Tens of thousand　 　　　　　　B. Tens of thousands of

C. Ten of thousands of　 　　　　　D. Tens and thousands

39. ________ you make a promise, you should keep it.

A. Though　　　 B. Unless　　　 C. Once　　　 D. Even if