You expect that continued increases in the frequency with which customers are targeted with email advertisements will lead to customer frustration and a reduction in salesafter a certain point (i.e. an inverted U relationship between Sales and number of email advertisements (#EmailAds)).

Model A: Sales = a + b* #EmailAds

Model B: Sales=a+b* ln(#EmailAds)

Model C: Sales = a + b* #EmailAds + c* #EmailAds²

Model D: Sales = a +b* #EmailAds + c* ln(EmailAds)

From the above models, which would be best to use for estimation:

A.Model C

B.Model B

C.Model D

D.Model A