Your task here is to write a Python function that calculates the year $($as an integer$)$ one must be born by to have lived through some fraction of all emissions, either land, fossil, or both.

Make a Python function named what$\_$year$()$ that takes two named arguments, fraction, for the fraction of cumulative CO$2$

we are interested in$,$ and source, which can take on string values 'Fossil', 'Land', or 'Both'. Give defaults of $0\mathrm{.}5$ and Both for these arguments. Thus, you should declare your function like so:

def what$\_$year$($fraction $=0\mathrm{.}5,$ source $=$ 'Both'$)$:

This function should return the latest year one could be born in such that fraction of all CO$2$

from the indicated source was emitted in one's lifetime. So$,$ for example, what$\_$year$($fraction $=\mathrm{.}5,$ source $=$ 'Both'$)$ should return $1991.$