Disposable income(billions of dollars) Consumption expenditure (billions of dollars) 400 450 600 600 800 750 1000 900 1200 1050 23) Based upon the above table, if disposable income is $400 billion, saving equals
选项：

A: -$50 billion.
B: $0 billion.
C: $50 billion.
D: $100 billion.