Answer:
A) 2 g
Explanation:
According to the graph, the half-life of Co-57 is 10 days. In twenty days (the amount of time between Jan 1 and Jan 21), two half-lives have passed, and, therefore, the amount of the sample has decreased by 50% twice. After the first ten days, 4g remain; after the second ten days, only 2g are left.
