Question

why should a circular saw that does not have an upper blade guard never be used?
the trigger lock will not work properly.
the safety features will be disabled.
the exposed blade poses a danger.

Explanation:

The upper blade guard on a circular saw protects users from the exposed blade during operation. Without it, the blade is open - which can cause serious injury. The trigger lock and other safety features are not directly related to the presence or absence of the upper blade guard.

Answer:

The exposed blade poses a danger.