A、Trouble. B、Youthful feeling. C、Companionship. D、Pressure. B短义提到，交换生使老年人感到年轻，所以给他们带来的是年轻的感受，故B为答案。

Laura Straub is a very worried woman. Her job is to find families for French teenagers who expect to live with American families in the summer. It’s not easy, even desperate. "We have many children left to place: 40 out of 75," said Straub, who works for a Paris-based foreign-exchange program called LEC.[16]When exchange programs started 50 years ago, family life was more accommodating. For one thing, more mothers stayed home. But now, increasing numbers of women work outside the home. Exchange-student programs have struggled in recent years to sign up host families for the 30,000 teenagers who annually come from abroad to spend an academic year in the United States, as well as the thousands more who participate in summer programs. School systems in many parts of the U.S., unhappy about accepting non-taxpaying students, have also strictly limited the number of exchange students they accept. At the same time, the idea of hosting foreign students is becoming less appealing. In searching for host families, who usually receive no pay,[18]exchange programs are increasingly broadening their appeals to include everyone from young couples to retirees. "We are open to many different types of families," said Vickie Weiner, eastern regional director for ASSE, a 25-year-old program that sends about 30,000 teen-agers on a-cademic-year exchange programs worldwide.[19]For elderly people, exchange students "keep us young—they really do," said Jean Foster, who is hosting 16-year-old Nina Porst from Denmark.
16. Why were exchange students more welcome by families 50 years ago?
17. Why are exchange students no longer popular in the U.S.?
18. How do the exchange programs attract more hosting families?
19. What may exchange students bring to elderly hosting people?