SAT阅读理解,高人进!

SAT阅读理解,高人进!
SAT OG 186页,第6题.
Becoming the best is very (difficult. No matter if) you are trying to become a better athlete, student, or musician. 改括号中的部分.
正确答案是 difficult, (逗号）whether
我看上的选项是 difficult. （句号）Regardless of whether
difficult; whether or not
问三者有什么区别?出高分,要详细解答.谢过.
我在书中看过（巴郎吧？）Whether or not不是很书面最好改为Regardless of whether...主要是三者用在哪里呢？求例句，跪谢。