4 V 2 * Choose the correct verb in brackets and complete the text with affirmative and negative forms of could, can and will be able to. 1 Advances in Medical Science Two hundred years ago, medical science was very basic and doctors could perform (perform / suffer) only simple operations. They (cut/use) X-rays to see inside their patients until about 100 years ago. Doctors ? (cure / cause) most infections until the discovery of penicillin in 1928. Recently, there have been incredible advances in medical science. Scientists (experiment / clone) animals these days, but they (clone / prevent) humans yet. In the future, doctors 5 (operate / discover) on people using nanobots. People 6 – (sleep/live) longer, perhaps 150 years, but we? (live/transplant) forever! ​