Why are bacteria required in the nitrogen cycle? a. the xylem in plants cannot function without bacteria. b. plants need to pull nitrogen directly from the air. c. plant dna can only be transmitted by bacteria. d. nitrogen needs to be fixed before it is used by plants?

            The correct option is d ( nitrogen needs to be fixed before it is used by plants).

Reason:
             Nitrogen fixing bacteria play a key role in nitrogen cycle, because they may a connection for the flow of nitrogen between atmosphere and the soil. Air contain 78% N, but it can not be taken by living organisms in its free form. Nitrogen fixing bacteria live in the roots of plants capture this free N and convert it into nitrogenous compounds which are then easily taken by plants. 

Number one is sinkholes may form from water erosion