题目

WASHINGTON, Sept. 6 (Xinhua) -- U.S. President Barack Obama on Monday announced a new plan to renew and expand the nation's roads, railways and runways, in a bid (努力) to restart the sluggish economy and to help create jobs. Obama announced the billion-dollar infrastructure(基础设施) spending program at an event in Milwaukee, Wisconsin, marking the Labor Day holiday in the United States. The proposal calls for investments over six years, including rebuilding and modernizing 150,000 miles (241,350 kilometers) of roads, 4,000 miles (6,430 kilometers) of railways and 150 miles ( 241 kilome基因工程操作的第二步是基因表达载体的构建，基因表达载体的组成必须有(      ) A.目的基因、启动子 B.目的基因、终止子 C.目的基因、启动子、终止子 D.目的基因、启动子、终止子、标记基因