We use wood here as fuel in to passive

Usually (but not always), passive voice can include the actor, usually following the verb. ... Yes, this makes sense; therefore, it is a passive voice sentence. To make this sentence active, you will need to put the noun doing the action in the subject location of the sentence.

Explanation:

resent Infinitive   to keepto be kept

Perfect Infinitive   to have keptto have been kept

Present Participle   & Gerundkeepingbeing kept

Perfect Participle   having kepthaving been kept

(happy to help)

right angles are formed by perpendicular lines. if the lines would extend, to make two intersecting lines, they would form four right angles.