单项选择题
A．To warn the reader to stay away from them. B．To compare their voltage to that used in houses. C．To give an example of a living electrical generator. D．To describe a new source of electrical power.
A．should be introduced, for it really works at the school where he once studied B．should not be introduced, for it may cause disturbance C．should be introduced though it may not work effectively D．needn't be introduced as long as teachers don't smoke in front of children
A．more beneficial it will be to science B．more powerful will be its electrical charge C．easier it will be to find D．tougher it will be to eat