Concept:A disadvantage of human power is a limitation that makes it less efficient compared to other farm power sources.
Explanation:Human power means work done by people using their physical strength.
It is easily available and can be controlled without much cost.
It also brings about the least damage to tools and the farm environment.
However, human energy and endurance are limited.
A person becomes tired quickly and cannot work for long hours at high speed.
Compared to animal, mechanical, or electrical power, human output is very small.
Thus, the key disadvantage is that human power has low output or low work capacity.
Options A, B, and C describe advantages rather than disadvantages of human power.
Answer:D. has low output