What human activity is most responsible for the damage to the ozone layer? a) the use of freon and cfc spray propellants. b) deforestation and interruption of the water cycle. c) the contamination of groundwater with mercury and pcbs. eliminate d) burning of fossil fuels and the subsequent creation of acid rain.

A) The use of freon and CFC spray propellants.

Chlorofluorocarbons (CFCs) are compounds  that are used as propellents in the spray cans and in refrigerators. These are manufactured under the trade name of Freon. Freon is a CFC that contains fluoride as a main ingredient. These CFCs are released in the atmosphere and they accumulate in the stratospheric layer. The ozone layer is present in the stratosphere shields the Earth from the harmful ultraviolet rays. The CFC molecules breaks down by the ultra violet radiations. The released chlorine atoms or fluorine atoms reacts with the ozone molecules resulting in the removal of ozone layer from the stratosphere. This is called as ozone layer depletion.  

the answer that best describes the relative refractory period in the neuron is the resting membrane potential is restored, but concentrations of sodium and potassium are not restored to their original state.

the refractory period is the time during which another stimulus given to the neuron will not lead to a second potential. thus, because na+ channels are inactivated during this time, additional depolarizing stimuli do not lead to new action potentials.  

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