题干

如图,两根光滑的金属导轨,平行放置在倾角为θ的斜面上,导轨的下端接有电阻R,导轨自身的电阻可忽略不计.斜面处在一匀强磁场中,磁场方向垂直斜面向上.质量为m、电阻可不计的金属棒ab,在沿着斜面与棒垂直的恒力F作用下沿导轨匀速上滑,并上升h高度.在这过程中(     )

A:作用于金属棒上的各力的合力所做的功等于零

B:作用于金属棒上的各力的合力所做的功等于mgh与电阻R上产生的焦耳热之和

C:恒力F与安培力的合力所做的功等于零

D:恒力F与重力的合力所做的功等于电阻R上产生的焦耳热

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A,D

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    Most people agree that honesty is a goodthing. But does Mother Nature agree? Animals can't talk, but can they lie in other ways? Can they lie with their bodies and behavior? Animal experts may not call it lying, but they do agree that many animals, from birds to chimpanzees, behave dishonestly to fool other animals. Why? Dishonesty often helps them survive.

    Many kinds of birds are very successful at fooling other animals. For example, a bird called the plover sometimes pretends to be hurt in order to protect its young. When a predator(猎食动物)gets close to its nest, the plover leads the predator away from the nest. How? It pretends to have a broken wing. The predator follows the "hurt" adult, leaving the baby birds safe in the nest.

    Another kind of bird, the scrub jay, buries its food so it always has something to eat. Scrub jays are also thieves.They watch where others bury their food and steal it. But clever scrub jays seem to know when a thief is watching them. So they go back later, unbury the food, and bury it again somewhere else.

    Birds called cuckoos have found a way to have babies without doing much work. How? They don't make nests(鸟巢). Instead, they get into other birds' nests secretly. Then they lay their eggs and fly away. When the baby birds come out, their adoptive parents feed them.

    Chimpanzees, or chimps, can also be sneaky. After a fight, the losing chimp will give its hand to the other. When the winning chimp puts out its hand, too, the chimps are friendly again. But an animal expert once saw a losing chimp take the winner's hand and start fighting again.

    Chimps are sneaky in other ways, too. When chimps find food that they love, such as bananas, it is natural for them to cry out. Then other chimps come running. But some clever chimps learn to cryvery softly when they find food. That way, other chimps don't hear them,  and they don't need to share their food.

    As children, many of us learn the saying "You can't fool Mother Nature." But maybe you can't trust her,either.