For years, consumers held the belief that SUVs $($sports utility vehicles$)$ were gas$-$guzzlers. That was, until automakers like Honda and Toyota began introducing hybrid models and low$-$cylinder models with proven fuel economy. The introduction of these hybrid SUVs had what sort of effect on buyers?

a$.$ Added a new belief by targeting marketing efforts

b$.$ No effect at all, as attitudes can never be changed

c$.$ Reduced the importance of a product attribute

d$.$ Turned a negative belief about a product attribute into a positive