For clarity, the pronouns this, that, which, and it should ordinarily refer to specific antecedents rather than to whole ideas or sentences. When a pronoun’s reference is needlessly broad, either replace the pronoun with a noun or supply an antecedent to which the pronoun clearly refers.

The writer substituted the noun ads for the pronoun this, which referred broadly to the idea expressed in the preceding sentence.

The writer added an antecedent ( fact) that the pronoun that clearly refers to.