Read the sentence. I’d like to study biology or math, but it will be difficult.

Which sentence corrects the vague pronoun problem in this sentence?

I’d like to study either one, but it will be difficult.
I’d like to study biology or math, but they will be difficult.
I’d like to study them, but making a choice between the two will be difficult.
I’d like to study biology or math, but choosing between the two will be difficult.