Multi-choice problem: Continue writing the next sentence in this paragraph:

How to save frequent flyer miles about to expire
Shop through an airline portal.
Almost every airline has online malls or portals where you can shop. You'll earn extra miles on your purchases, and the clock will " reset, " so you won't lose any miles because of inactivity.

I. Airlines also have websites that you can look through. Go to airbnb or call faqs if you need to confirm your trip. II. It's especially important to shop at airports you plan to frequent online. Pay attention to the signs, as they will tell you how many miles you need to travel to purchase your ticket. III. Many airlines list their malls on their websites. However, while some airlines sell their malls here, others charge for a trip to one of them. IV. To shop, visit your airline's portal and search for a store. Then click through to the retailer's website when you are ready to make your purchase.
IV.