题目内容：

Booking a trip on an online travel site is convenient, but comes with its own set of problems.

◆They know who's going to spend more.

Last year, US travel research company Orbitz tracked people's online activities to test out whether Mac users spend more on travel than PC users. It turns out that on average, Mac users lay out US $20-30 more per night on hotels. As a result, online travel sites show these users more expensive travel options first. To avoid paying more, sort results by price.

◆Their software doesn't always link to the hotel's system.

A guaranteed reservation is almost impossible to come by anywhere — but the risk of your flight or hotel being overbooked increases with third-party providers. The middleman's software isn't immune(防护作用的) to system errors, so always call the hotel or airline to make sure your booking was processed.

◆Don't be fooled by packages: Often, they're low-end items grouped together.

Ever notice how travel sites recommend a hotel, a rental car, and tour package all in one click? These deals usually feature travel that no one wants, like flights with multiple layovers(中途停留). Check the fine print.

◆Once your trip is purchased, you're on your own.

An online travel agency can't provide assistance the same way an agent can if a flight is cancelled or a room is substandard. Basically, when you arrive at the airport or hotel, you're just another customer who booked at the lowest rate.

1.What does the writer mainly intend to tell readers?

A. Steps of reserving a hotel on line

B. Tips of how to reserve flights on line

C. Problems about reserving a trip on line

D. Traps in reserving hotels and flights on line

2.Why do people probably spend more on their online reservation for trips?

A. They are shown less expensive travel options first

B. Some of the travel expenses are earned by Mac users

C. They book at prices recommended first by travel sites.

D. Their flights or hotels are overbooked by tourist agencies

3.What can we lean from the passage?

A. Booking online is better than booking in other ways

B. The third parties will provide good travel services on line

C. Few people book trips on line because of possible troubles

D. Travelers’ interests may not be guaranteed with booking on line.