题目

根瘤菌与大豆的根共生，根瘤菌的作用是 A.固氮                  B.提供二氧化碳      C.提供水分           D.提供无机盐 答案：AI have ________ finished my homework but he hasn't _________.A.yet; alreadyB.ever; neverC.already; yetD.already; still