Swimmers must be very strong in order to build up speed because of the high resistance of the water. Runners, ____, must struggle against the heavy resistance of the air and ground.
"Perhaps we need to send for Dr. Smith to see what we can do about it, " Father suggested __________ to his neighbor who had come to discuss the problem.
(2009·湖北) As there is less and less coal and oil, scientists are exploring new ways of making use of __________ energy, such as sunlight, wind and water for power and fuel.