I can't see any explanation for this question.

I'm currently able to see answer explanations when I choose an answer then review the test. I'm on mobile. Is it still not showing for you?

Also sorry, realized I never answered your question: I would look around lines 42-45, in which we are told that under the English system, a rule will be formally applied unless there is some uncertainty as to its validity, otherwise substantive reasoning may prevail.