@Cheng5 if she sent you an Inquiry, and you responded with your question about the reason for her wanting to rent your place, that's all you need to do. In fact, you don't have to Pre-Approve or Decline. You can dismiss it.
If the message was sent as a Reservation Request, and the guest hasn't answered your question in a reasonable amount of time, I would decline with a polite note that she's welcome to send you an Inquiry where you can discuss the matter further. Declining a reservation request does not affect your response rate unless you decline an excessive number of Reservation Requests. Best to do this rather than letting the request expire...if it expires, Airbnb will block the dates.