Read the following passages and choose the best answer to each of the questions following the passage. Then write the corresponding letter in the brackets.Passage One As every physics student learns, there are four known forces of nature: gravity, electromagnetism, a "strong force" that binds atomic nuclei and a "weak force” that governs certain types of radioactive decay. Last week researchers at the Los Alamos National Laboratory announced that they may have found the best evidence yet for a hypothetical, elusive "fifth force”. If confirmed, their findings could mean that Sir Issac Newton's famous inverse-square law of gravity is in danger of losing the dignified position it has held for three centuries. “It's like saying Mem and apple pie's no good any more”, admits the leader of the gravity project, Geophysicist Mark Ander. “You just don't do that lightly.” The physicists reached their conclusion as the result of an experiment conducted in Greenland last summer. They lowered a supersensitive meter into a mile-deep hole bored in glacial ice--chosen because its density is more uniform than that of rock—and monitored the gravity as the meter descended, What occurred was startling: the expected increase in gravitational force predicted by Newton was there, but it got stronger, faster than expected. Either something was enhancing the force of gravity or the researchers had come upon a heretofore unknown, far more complex working of gravity itself. Or, just possibly, they had made a mistake. The fifth force, if that is what it is, has been a source of debate among physicists since its existence was suggested in 1981 by Australian mineshaft experiments. Five years later, Purdue University Physics Professor E. Fischbach measured a weak force he called "hypercharge" and theorized that it caused object of different composition to fall at different rates. Since Fischbach's finding, as many as 45 experiments have sprung up in search of the mystery force, and so far each has served only to confuse rather than clarify the issue. Jim Thomas, a physicist at the California Institute of Technology, praises the technical precision of Ander's experiment, but cautions that measuring gravity in holes is inexact at best. He points out, for example, that an aberration in the earth's crust might have caused the unusual measurements. What we are talking about is the possible modification of gravity, which is the fourth force,"adds Thomas. Even Ander stresses the strict confirmation is needed before he accepts the results of his Greenland experiment, Says he, "Gee, I've gotta be wrong—Newton certainly can't be wrong!"