First-class levers .

a. are used when standing on tip-toe
b. are typified by tweezers or forceps
c. have load at one end of the lever, fulcrum at the other, and effort applied somewhere in the middle
d. in the body can operate at a mechanical advantage or mechanical disadvantage, depending on specific location

d. in the body can operate at a mechanical advantage or mechanical disadvantage, depending on specific location.

Explanation:

A first-class lever is defined as the force in which fulcrum is present in the center and the load and effort on each side of the fulcrum. First class lever moves over a large distance to move the load to a smaller distance.

First-class lever can be operated at mechanical advantage or disadvantage that it can aid in work by changing the direction of the force and by increasing the efficiency. As the the fulcrum is between the load and the effort so the length of the effort arm divided by the length of the effort arm.

If the fulcrum is closer to the effort, it will operated at mechanical disadvantage and vice versa.

Example of first class levers includes scissors, a claw hammer, a see-saw and a weighing balance.

Hence, the correct option is D.

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