<p>The question is :</p>
<p>After reading about a series of local break-ins, the Levy family believed that they had purchased a security system that would have prevented any intrusion.</p>
<p>I knew 'would have' wasn't correct but after cutting down the option choices, I was stuck between the following :</p>
<p>a.) would prevent any intrusion
b.) will prevent any intrusion</p>
<p>I chose 'will' but it turned out that the correct answer was 'would'. I am pretty confused here as there was virtually no way I could have deduced the correct answer. Am I missing a grammar rule ? What's the general consensus about the usage of will vs would ?</p>