How many of the following experiments must be exponential random variables?

i.An experiment which records the time it takes for all $50$ iPhone batteries in a shipment to lose their charge.

ii.An experiment which records the number of full days an iPhone needs without a reset.

iii.An experiment which has probability density function equal to $f(x)=5e^{5x}$ for $x<0$.

iv.An experiment whose outcomes are all positive real numbers.

v.An experiment which measures how long heart-attack patients survive after hospital discharge.