Half-life is the time taken for the concentration of the substance to reduce by 50%. The original sample of strontium had a mass of 1.6 gms. Thus, option d is correct.
<h3>What is half-life?</h3>
The half-life of any radioactive substance is the time period at which the concentration will get reduced to half the initial amount. The initial mass of Sr-90 is calculated as,

Given,
Quantity of the remaining substance N (t) = 0.40 gm
Initial radioactive substance quantity
=?
Time duration (t) = 56 years
Half-life = 28 years
Substituting values above:

= 1.6 gm
Therefore, option d. the initial mass of Sr is 1.6 gm.
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To make clay float in water you have to shape it into something wide enough and roughly bowl-shaped so that it can displace enough water to compensate for its mass, just like a real boat.
I believe it is C but if not that A
One atom is a metal and one is a nonmetal.
One atom has high electronegativity value, while the other value is really low