====Benjamin Franklin established that lightning is the transfer of positive or negative electrical charge between regions of a cloud or from cloud to earth. Such transfers require that electrically neutral clouds, with uniform charge distributions, become electrified by separation of charges into distinct regions. The greater this separation is, the greater the voltage, or electrical potential of the cloud. Scientists still do not now(KNOW?) the precise distribution of charges in thunderclouds nor how separation adequate to support the huge voltages typical of lightning bolts arises. According to one theory, the precipitation hypothesis, charge separation occurs as a result of precipitation. Larger droplets in a thundercloud precipitate downward past smaller suspended droplets. Collisions among droplets transfer negative charge to precipitating droplets, leaving the suspended droplets with a positive charge, thus producing a positive dipole in which the lower region of the thundercloud is filled with negatively charged raindrops and the upper with positively charged suspended droplets.
17. The passage is primarily concerned with discussing which of the following?
(A) A central issue in the explanation of how lightning occurs
(B) Benjamin Franklin’s activities as a scientist
(C) Research into the strength and distribution of thunderstorms
(D) The direction of movement of electrical charges in thunderclouds
(E) The relation between a cloud’s charge distribution and its voltage
18. The passage suggests that lightning bolts typically
(A) produce a distribution of charges called a positive dipole in the clouds where they originate
(B) result in the movement of negative charges to the centers of the clouds where they originate
(C) result in the suspension of large, positively charged raindrops at the tops of the clouds where they originate
(D) originate in clouds that have large numbers of negatively charged droplets in their upper regions
(E) originate in clouds in which the positive and negative charges are not uniformly distributed
19. According to the passage, Benjamin Franklin contributed to the scientific study of lightning by
(A) testing a theory proposed earlier, showing it to be false, and developing an alternative, far more successful theory of his own
(B) making an important discovery that is still important for scientific investigations of lightning
(C) introducing a hypothesis that, though recently shown to be false, proved to be a useful source of insights for scientists studying lightning
(D) developing a technique that has enabled scientists to measure more precisely the phenomena that affect the strength and location of lightning bolts
(E) predicting correctly that two factors previously thought unrelated to lightning would eventually be shown to contribute jointly to the strength and location of lightning bolts
20. Which of the following, if true, would most seriously undermine the precipitation hypothesis, as it is set forth in the passage?
(A) Larger clouds are more likely than smaller clouds to be characterized by complete separation of positive and negative charges.
(B) In smaller clouds lightning more often occurs within the cloud than between the cloud and the earth.
(C) Large raindrops move more rapidly in small clouds than they do in large clouds.
(D) Clouds that are smaller than average in size rarely, if ever, produce lightning bolts.
(E) In clouds of all sizes negative charges concentrate in the center of the clouds when the clouds become electrically charged.