学校把载750棵树的任务，按照人数的比例分配给三个年级．四年级有40人，五年级有42人，六年级有43人．每个年级各分多少棵？

It is reported that China will not buy the Euro debt until some thorough research ______.

A．will be done

B．had been done

C．has been done

D．will have been done