What Was the Serious Flaw in Early Studies That Supported

What was the serious flaw in early studies that supported claims that bilingualism stunts cognitive development?

A) The monolingual students prepared in advance for the tests.
B) The bilingual students were given reduced bonus rewards.
C) The bilingual students were immigrants whose poor command of English gave them a disadvantage during the tests.
D) The bilingual students spoke complex languages such as Finnish that place great demands on working memory.