When ethnic groups intermarry, it is likely to result in cognitive benefits; indeed, perhaps already having contributed to the Flynn effect, which has been the worldwide rise in average IQ score by as much as three points per decade over most of the past century.
A)When ethnic groups intermarry, it is likely to result in cognitive benefits; indeed, perhaps having contributed to the Flynn effect, which has been
B)The intermarriage of ethnic groups is likely to result in cognitive benefits and may indeed have contributed to the Flynn effect,
C)When ethnic groups intermarry, they are likely to result in cognitive benefits and may indeed have contributed to the Flynn effect, the name given to
D)Ethnic groups that intermarry will likely result in cognitive benefits; indeed, possibly having contributed to the Flynn effect,
E)The intermarriage of ethnic groups, likely to result in cognitive benefits, indeed have possibly contributed to the Flynn effect, the name given to

OA is B and to me also it appears to be correct answer.
But I am a little skeptical over the use of the verb "has" here. I believe the subject here is "Intermarriage of ethnic groups" which is singular, so by rule I was first expecting to have a "has" in the answer but when comparing to a sentence like "she might have" and "she might has", obviously the former sounds better. Also, in absence of might "she has" is correct and not "she have". Probably I am missing out on some concept. Can anyone help me analyse this?