(c) is correct because, as explained in the web page, almost half of the newborns with sickle-cell disease died by the age of l5, thus seriously skewing the statistics toward a lower number of adults with the condition. Athough (a) is a possibility (and was actually suggested at the time) there was no concrete evidence for it at all (the effects produced by a given gene can be altered considerably by what other genes are present in the organism, and may, in fact, be the cause for such variation in phenotypic expression in sickle-cell disease as have been observed by clinicians). (b) is also a logical possibility, but since most U.S. African-Americans did in fact come from West Africa in the area surrounding the Gulf of Guinea, the gene frequencies in the U.S. black population are not likely to have been very different when the U.S. population was established during the slave trade.