Spontaneous depolarization of the nodal tissue in the sa node occurs at action potentials per minute:

It is 70-80 action potentials per minute.  The rate of phase 4 depolarization in the SA node sets the Heart rate, The SA node serves as the Pacemaker It has the greatest  automaticity And ability to  spontaneously generate an action potential.  

the cost of mining is extremely low compared to the money earned from mining, creating income to improve the environment.

c is the answer to this question. altitude affects the weather because air molecules evaporate.