英语阅读题,Directions:After reading the passage,you are required

英语阅读题,
Directions:After reading the passage,you are required to complete the statements that follow the questions (No.56 through No.60).You should write your answers in not more than 3 words on the Answer Sheet correspondingly.
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 simply be sealed against the vacuum space,a submersible must be able to bear extreme pressure if it is not 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 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 vehicle---particularly remote-operated vehicles---operated at the end of long ropes.
For a similar reason,knowing where you are undersea is much more difficult than in space.A spaceship’s position can be located by following its radio signal,or by using telescopes and radar.For an undersea vehicle,however,a special network of sonar devices must be laid out in advance on the ocean floor in the area of a dive to locate the vehicle’s position.
Though undersea exploration is more challenging than outer space in a number of respects,it has a distinct advantage:Going to the ocean depths doesn’t make it necessary to escape the Earth’s gravity.Thus,it remains far less expensive.
56.Why didn’t people begin to use unmanned vehicles in undersea exploration until recently?
It is because of the _______________________.
57.What does the author mean by saying “more challenging” in the last paragraph?
____________________________________________.
58.Why is ocean exploration not so expensive as space travel?
Because ______________________ for submersibles to get away from the Earth’s gravity.
59.When must the sonar devices be placed?
Before the _________________________.
60.What will happen to a submersible in deep water if it is not well built?
It is most likely to ___________________________________.