What Has Research Found to Be the Most Effective Way

What has research found to be the most effective way to teach English to children of Hispanic heritage while simultaneously improving their Spanish?

A) providing an English-only curriculum at school and encouraging the use of Spanish at home
B) providing the majority of instruction in Spanish, with a small amount of time reading in English each day
C) combining English and Spanish instruction
D) completely immersing the children in the English language and discouraging the use of Spanish at home