ANIMAL EXPERIMENTS SUGGEST THAT GOOD NUTRITION DURING THE FIRST THREE...

Câu 20: Animal experiments suggest that good nutrition during the first three years of human life is crucial.A. Experiments have proven that it is very important for a human baby under three years of age to have some nutrition.B. Because of the experiments with animals, researchers believe that good nutrition for infants is very important.C. If animals are deprived of health food for three years, they will not be healthy.D. If infant humans do not eat good food for three years, they will act like animalsĐọc kỹ đoạn văn sau và chọn phương án đúng (A, B, C hoặc D)Laser is an acronym for light amplification by stimulated emission of radiation. Stimulated emission is a variation ofspontaneous emission, a process that occurs in atoms when an electron in a ground, or unexcited state, is knocked into ahigher state when energy is applied to the system. As the electron drops back into ground state, a photon, or particle oflight, is released. As de-excitation occurs in millions of atoms, photons are released in a random fashion, and light isemitted in every direction. Stimulated emission, however, causes an increase in the number of photons traveling in aparticular direction. An optical cavity, the space formed by two reflective surfaces facing each other, is used to controlthe direction of the beam. There are solid-state, gas, and liquid lasers, and by subjecting lasing materials to various typesof energy- electrical, magnetic, or sonic- scientists have been able to control the laser output to suit various functionsand applications.In industry, the laser has proven to be a very versatile tool, particularly for cutting and welding. Lasers are now alsoused in high-speed printing and in the creation of three-dimensional images, called holograms. Laser tracking andranging systems have been developed, using light signals to measure distance rather than the radio signals of radar. Theuse of the laser in biological and medical applications is also rapidly expanding, and the laser is already being used withgreat success in certain surgical procedures. In the field of communications the laser, used in conjunction with fiber-optic networks, is capable of carrying much more information than conventional wires and is setting the stage for the“electronic superhighway” of the near future.