The nucleus controls what happens within the cell 
 
        
                    
             
        
        
        
Neurotransmitters can affect postsynaptic cells by causing molecular changes in the cells.
The term "chemical messengers of the body" is frequently used to describe neurotransmitters. The nervous system uses these molecules to send information between neurons or from neurons to muscles. A neurotransmitter influences a neuron in one of three ways: excitatory, inhibitory or modulatory. 
When a stimulus is potent enough, the chemical messenger neurotransmitter is transferred into the nerve impulse. If it is powerful enough, it will reach the postsynaptic cell and affect the structure of the neuron. And when signal transduction happens, everything mentioned above is completed.
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Lactic acid buildup when you’re exercising causes a build up of oxygen. Your body cannot break down glucose in the blood. So, your heart beat increases because there is more oxygen than normal needing to be pumped throughout the body. 
In short terms: because your heart needs to pump more oxygen, it is pumping faster causing a faster heart rate.
        
             
        
        
        
The answer for your question is A.