一道SAT的语法题

题目

一道SAT的语法题
New Zealand had fostered a thriving local music scene for decades,but it was not until the late 1980s (when) New Zealand musicians began to reach international audiences.答案是When,为什么When是错的?

答案

...it was not until the late 1980s ( ) New Zealand musicians began to reach international audiences.这个空格应该填that,因为" It was...that..."是强调句,本句强调了“not until the late 1980s”.不强调的话...