What Has Research Determined Is the Difference in English Fluency

What has research determined is the difference in English fluency between children born in the United States and children brought to the country before the age of four?

A) Children born in the United States had a significantly higher probability of being fluent in English than those who immigrated at a young age.
B) Children born in the United States had a slightly higher probability of being fluent in English than those who immigrated at a young age.
C) Children born in the United States had no discernible difference in English fluency than those who immigrated at a young age.
D) Children born in the United States had no discernible difference in English fluency than those who immigrated at a young age when their parents were proficient English speakers.