In a simple 300 million-person world of all four-person families, if the poverty line is $12,500 and half of the 10 million families (with 40 million poor people) earn $10,000 and the other half earn $7,500, then the poverty gap is

a. $125 billion (=10 million * $12,500).

b. $250 billion (=20 million * $12,500).

c. $150 billion (=20 million * $2,500 + 20 million * $5,000).

d. $37.5 billion (=5 million * $2,500 + 5 million * $5,000).