Explanation:
Atomic radius decreases from left to right in a period.
Therefore Calcium would have a smaller atomic size.
Answer chocies .......................
It's A. volume
Pressure =

with const depends on the chosen unit of volume
I think so...
Cs -137 has a half life of about 30 years. If 60 years pass, there is two half lives passed so 1/2 * 1/2= 1/4. Take 1/4 and multiply it with the mass given:
(1/4)*20mg=5mg left