13.Y has been believed to cause Z. A new report， noting that Y and Z are often observed to be preceded by X， suggests that X， not Y， may be the cause of Z.Which of the following further observations would best support the new report's suggestion？

　　（A） In cases where X occurs but Y does not， X is usually followed by Z.

　　（B） In cases where X occurs， followed by Y， Y is usually followed by Z.

　　（C） In cases where Y occurs but X does not， Y is usually followed by Z.

　　（D） In cases where Y occurs but Z does not， Y is usually preceded by X.

　　（E） In cases where Z occurs， it is usually preceded by X and Y.

　　14.A cost-effective solution to the problem of airport congestion is to provide high-speed ground transportation between major cities lying 200 to 500 miles apart. The successful implementation of this plan would cost far less than expanding existing airports and would also reduce the number of airplanes clogging both airports and airways.

　　Which of the following， if true， could proponents of the plan above most appropriately cite as a piece of evidence for the soundness of their plan？

　　（A）An effective high-speed ground-transportation system would require major repairs to many highways and mass-transit improvements.

　　（B）One-half of all departing flights in the nation's busiest airport head for a destination in a major city 225 miles away. （C）The majority of travelers departing from rural airports are flying to destinations in cities over 600 miles away.

　　（D） Many new airports are being built in areas that are presently served by high-speed ground-transportation systems.

　　（E） A large proportion of air travelers are vacationers who are taking long-distance flights.

　　15.Which of the following， if true， would most strengthen the conclusion drawn in the second sentence？

　　（A） Commercial airliners are already required by law to be equipped with extremely sophisticated radar systems.

　　（B） Centrally located airports are experiencing over-crowded airspace primarily because of sharp increases in commercial-airline traffic.

　　（C） Many pilots of private planes would rather buy radar equipment than be excluded from centrally located airports.

　　（D） The number of midair collisions that occur near centrally located airports has decreased in recent years.

　　（E） Private planes not equipped with radar systems cause a disproportionately large number of midair collisions around centrally located airports.