If an engine cannot be 100% efficient, what happens to the
energy that is lost?
A. It disappears.
B. It reproduces.
C. It still exists, but in a different form.

Given the evidence in the explanation, I'm pretty sure it's C. It still exists, but in a different form.

Explanation: "Some part of the energy supplied is used to change the internal energy of the system. Some part is also released into the surroundings. Generally, frictional losses are more predominant for the machines being not 100% efficient. This friction leads to the loss of energy in the form of heat, into the surroundings."

1 a

2 b

3 b

4 d

5 d

6 d

explanation: