[单项选择]The major difficulty in developing space travel is that ______.
A. there is no clear destination in space
B. it is too risky and expensive for people to go beyond the orbit
C. the current space propulsion systems can not meet the needs for the long space travel
D. there is not enough energy for spacecraft to gain velocity

[单项选择]The major difficulty in developing space travel is that ______.


A. there is no clear destination in space
B. it is too risky and expensive for people to go beyond the orbit
C. the current space propulsion systems can not meet the needs for the long space travel
D. there is not enough energy for spacecraft to gain velocity

[单项选择]The passage suggests that one major difficulty in establishing patterns of migration is
A. (A) the infrequency of life course events, which restricts the amount of data available to demographers
B. (B) the overwhelming availability of proxies in migration patterns, which creates a degree of stasis in migration records
C. (C) the lack of parishioner birth records, which limits the hard evidence upon which demographers base their observations
D. (D) the homogeneity of single parishes, which makes it difficult to distinguish the motivations of migrants
E. (E) the repetitive nature of migration, which results in a surfeit of unusable data that overwhelms demographers

[单项选择]The travel industry plays a major role in the ______ of business, recreation, and family life.


A. sphere
B. scales
C. surplus
D. surveys

[单项选择]______ I a billionaire, I would travel in space like an astronaut.
A. If only
B. Wishing
C. Were
D. Should

[单项选择]A major goal of the state travel agency is to ______more people to visit its country at the turn of the century.
A. reduce
B. expect
C. arouse
D. induce

[单项选择]Although the hazards of space travel are many, tile rewards made it a worthwhile ______.
A. endeavor
B. execution
C. expenditure
D. expedition

[单项选择]

As in the field of space travel, so in undersea exploration new technologies Continue to appear. They share a number of similarities with each other -- as well as some important differences.
Manned submersibles, like spaceships, must maintain living conditions in an unnatural environment. But while a spaceship must be able to bear extreme pressure if it is not to break up in deep water.
In exploring space, unmanned vehicles were employed before astronauts. In undersea exploration, on the other hand, men paved the way, and only recently have unmanned remote-operated vehicles (ROVs) been put to use.
One reason for this is that communicating with vehicles in orbit is much easier than talking to those underwater. A vacuum is an ideal medium for radio communications, but underwater communications are limited to much slower sound waves. Thus, most undersea vehicles -- particularly ROVs -- operate at the end of long ropes.
For a similar reason, knowing wh
A. the communication problem
B. the ocean depths
C. the movement of waves
D. the problem of vacuum