The color scheme shown here would be an appropriate choice for a $\_\_\_\_\_\_\_\_\_\_.$

The colors shown vary by shades $($from left to right they are darker and then fade to lighter hues$)$ indicating order.

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a$.$ categorical variable having groups with inherent ascending or descending order

b$.$ quantitative variable in which the magnitudes of value do not need to be expressed

c$.$ categorical variable having groups with no inherent ascending or descending order

d$.$ quantitative variable in which the magnitudes of value need to be expressed