Read the passage and choose the correct answer from a, b, c or d:
It’s amazing and difficult to believe, but we’re actually less familiar with the depths of the
oceans on our own planet than we are with the surface of Mars. Robots are now being used
increasingly to carry out the task of gathering data about the world below the waves. These
specially designed robots are able to obtain data more quickly and efficiently than human beings.
However, many of the robots, currently being used for this purpose, require batteries which run
out after a certain amount of time underwater and, therefore, they need to return to be
recharged regularly. As a result, they cannot stay underwater long enough to reach the deepest
points of the ocean, yet these tend to be the areas that scientists are most keen to learn more
Consequently, a lot of excitement has been produced by a technological innovation which
involves using the energy generated by the robot’s movement from warmer to colder waters and
the other way round. All the robot has to do is move through the sea and enough energy is
generated to charge its battery at no cost whatsoever, and the result is that data can be collected
for significantly longer periods of time.
It looks as though this kind of technology will increase the amount of the ocean that
underwater expeditions can investigate by allowing them to do this for much longer periods of
time. Nevertheless, scientists who like to create a map of the whole ocean still face massive
challenges when it comes to the logistics and finances that are necessary for the completion of
this task. Some estimates have suggested that even with innovations such as the one described in
this text, at least 3,000 robots would have to be in operation for a period of ten years in order to
map the whole ocean.
43. What is the disadvantage of the robots mentioned in the first paragraph?