“The impact will only grow     Iran’s leaders decide to change course and meet their international obligations,” US Secretary of State Clinton warned．

A．in case that

B．provided that

C．unless

D．until

水稻是喜温作物，在每年的三四月份育秧时，为了防止霜冻，傍晚常常在秧田里灌一些水过夜，第二天太阳升起后，再把秧田里的水放掉，为什么?