Until recently, the Zika virus was a rare and mild public health concern and little was known about it. In early 2015, Brazil reported an epidemic of Zika viral infection. Reports soon followed of unusual numbers of neurological disorders such as microcephaly in newborns, a condition characterized by a very small head and poor brain growth.

(a) Explain why we cannot conclude from this information alone that Zika infections cause microcephaly in newborns. Select the correct choice from the options.

a. Some Brazilians are known carriers of the Zika virus, and this prevents us from drawing any conclusions.
b. Correlation and regression lines describe only linear relationships, so no conclusions can be drawn.
c. There may be a lurking variable that is causing an increase in both Zika infections and microcephaly in newborns. It may not be Zika virus itself causing the increase of microcephaly in newborns.
d. We can conclude causality due to the sheer size of the study.