$8$

$08.$ A concave response relationship between a marketing investment $($e$.$g$.,$ every dollar spent on advertising$)$ and a marketing outcome $($e$.$g$.,$ every dollar earned in profits$)$ indicates that

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a$.$ the outcome increases with greater marketing investment but at a diminishing rate

b$.$ the outcome will always increase as marketing investment increases

c$.$ at low level of investment, marketing is ineffective because the level of investment is insufficient to compete, but at a high level of investment, marketing is also ineffective due to saturation

d$.$ all of the above