A city funds its public good provision solely from voluntary contributions from its two residents, Mr. A and Mrs. B. The total amount of spending on public good is equal to the sum of private contributions on public good, $g_A$ and $g_B$, divided by the marginal cost of public good: $G = (g_A + g_B)/c$. Assume that the marginal cost of producing public good is equal to 1 unit of income and thus we can simplify the formula for the total level of public good spending to just the sum of individual voluntary contributions: $G = g_A + g_B$. It is known, from the income tax records, that A's income is $m_A = 100$, B's income is $m_B = 1000$. Each of the two residents has a unitality function that shows preferences over a public good, G, and a private good, z: $U(z_A, G) = 0.3 \cdot ln(z_A) + 0.7 \cdot ln(G)$ and $U(z_B, G) = 0.8 \cdot ln(z_B) + 0.2 \cdot ln(G)$. Suppose Mrs. B donates nothing (remember that we are modeling voluntary contributions). How much does Mr. A donate in this case?
A. 0
B. 30
C. 70
D. 100