Question

Garbage causes pollution. It may be used as a source of energy (complex)​

Answer 1

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Garbage which causes pollution may be used as a source of energy.

Explanation ↴

We have to use an adjective clause to make it Complex sentence because an adjective clause (also called relative clause) is a dependent clause that modifies a noun or pronoun.

It tells which one or what kind. Adjective clauses almost always come right after the nouns they modify.

Answer 2

Garbage which cause pollution may be used as a source of energy.

☇Hope it helps☇