Can you explain this algebraically (even if you also give a graphed explanation)?

Radioactive substances decay over time. the mass M, in grams, of a particular radioactive substance d days after the beginning of an experiment is shown in the table below:

Number of days, d Mas, M (grams)
0 120.00
30 103.21
60 88.78
90 76.36

If this relationship is modeled by the function M(d) = a • 10^bd, which of the following could be the values of a and b?

A) a = 12 and b = 0.0145

B) a = 12 and b = -0.0145

C) a = 120 and b = 0.0022

D) a = 120 and b = -0.0022

PWN the SAT Math Guide p. 112 (p. 110 in later printing) #3:

Question relating to the exponent in the answer: It is a little unclear to me that the Value column indicates the value at the END of each year. The text states that the values represent “the value of his account in each of the next three years.” Must we presume that “in each” means at the END of each of those years, at which point the interest would have been accrued for that full year?