You Have Estimated the Following Equation $$\widehat { \text { TestScore } } = 607.3 + 3.85 \text { Income } - 0.0423 \text { Income } { } ^ { 2 } \text {, }$$

You have estimated the following equation: $$\widehat { \text { TestScore } } = 607.3 + 3.85 \text { Income } - 0.0423 \text { Income } { } ^ { 2 } \text {, }$$
where TestScore is the average of the reading and math scores on the Stanford 9 standardized test administered to $5 ^ { \text {th } }$ grade students in 420 California school districts in 1998 and 1999. Income is the average annual per capita income in the school district, measured in thousands of 1998 dollars. The equation
a. suggests a positive relationship between test scores and income for most of the sample.
b. is positive until a value of Income of $610.81$ .
c. does not make much sense since the square of income is entered.
d. suggests a positive relationship between test scores and income for all of the sample.