在根瘤菌与大豆的共生中，根瘤菌的作用是       (      )

A．固氮

B．提供水分

C．提供CO2

D．提供有机物

A project's payback period ends when （）.

A．profit maximum is realized

B．unit profit is realized

C．monthly revenue exceeds monthly costs

D．cumulative revenue equals cumulative costs