A、Too many planes. B、Weather. C、Different regions operate differently. D、All of the above. D短文中提到航班延误的问题，选项A、B、C都包括在内。

Have you ever had travel problems because your airplane was late? It is a common problem and it is getting worse. Airport delays make people angry and cost the country billions in lost work time.
    The Federal Aviation Administration (FAA) is the government agency that oversees air travel. They studied the problem of airline delays.
    They found eleven major problems and proposed eleven ways to fix them. Some of the problems are caused because different regions of the country do things differently. The regions don’t talk to each other enough. Some of them need new computers.
    Also, more planes are flying. A strong economy has more people using airplanes to get around. Airplanes are a form of mass transportation now, like trains and buses were in the past. More planes in the airport cause delays.
    The airport in San Francisco is a good example. It is growing fast. Many people go through San Francisco to get to other places around the world.
    Ron Wilson of the San Francisco International Airport says, "If you’ve got 18 flights that all want to take off at 8:00 a. m. and you’re on the 18th plane in line, you’re going to ’be 40 minutes late."
     Weather is another main reason for delays. Weather causes about 70% of delays. Fog or freezing rain can cause delays.
    The first thing the FAA wants to do is to take control away from the regions during heavy traffic times and bad weather. A national center would make decisions on things that affect the whole country. The FAA also wants to put more distance between planes in the sky when the weather is bad. This rule could cause even more delays.
    The FAA knows that their solutions will not cure the problems with airport delays. But, they hope to make things better. There are just too many planes, too few traffic controllers, and not enough new technology.