Choose the correct answer:
* ...................the doctor at 2:00 this afternoon so i can't go to the lecture.
A) I'm seeing
B) I see
C) I will see

What is the correct answer?
Is it A? Or C?
I know that both answers are in the future tense.. but why should we use one over the other?

Also, I would like to know when to use (Going to +Verb) rather than (Will + Verb)..

Thank you

Question

Choose the correct answer:
* ...................the  doctor at 2:00 this afternoon so i can't go to the lecture. 
A) I'm seeing
B) I see
C) I will see

What is the correct answer?
Is it A? Or C? 
I know that both answers are in the future tense.. but why should we use one over the other?

Also, I would like to know when to use (Going to +Verb) rather than (Will + Verb)..

Thank you

Answer

A) I'm seeing

You say this in the present, because it is clear from '2 o'clock' that it's in the future and it is a definite arrangement.

When you are making a decision use will; use going to after a firm decision has been made.
Often they can be used interchangeably with no difference in meaning. Even if you mix them up, a native speaker will understand you.

Answer

'A' is the correct answer here as it is a planned/arranged event.