What is the $SI$ unit of mass? How is it defined?

Provide the full forms of the following:
$(i)$ $AZT$
$(ii)$ $CFC$
$(iii)$ $SI$

From the following masses,the one which is expressed nearest to the milligram is (in $g$)

The melting point of ice in Fahrenheit scale is........

If the diameter of a carbon atom is $0.15 \,nm$,calculate the number of carbon atoms which can be placed side by side in a straight line across the length of a scale of length $20 \,cm$.

Which smaller units of $kg$ are used to measure the mass of chemicals with very low mass?