For a wind farm to be effective, there needs to be

a. many turbines

b. wind in only one direction

c. low average wind speeds

d. one turbine

The correct answer for the blank is-

A. many turbines .

In a wind farm, wind energy is harnessed in the form of mechanical energy, which is used to produce electricity when turbines rotate ( due to propeller like blades present around a rotor) due to higher speed of wind. The rotor is linked to the main body that rotates, ultimately leading to generation of electricity.

This means that larger the number of turbines, larger is the amount of energy harnessed.

Thus, for a wind farm to be effective, there needs to be many turbines.