The value of homes in our neighborhood has plummeted during the past three years. This has made it impossible for us to consider selling our house and moving closer to our aging parents.

The value of homes in our neighborhood has plummeted during the past three years. The uncertainty of the market has made it impossible for us to consider selling our house and moving closer to our aging parents.

The first answer has unclear reference: The pronoun This does not refer to anything specific in the first sentence. In the second answer, the vague This has been replaced by the specific subject The uncertainty of the market, so no problem of reference exists.

For more help, see section 23b.