Is the sentence "I will see the doctor at 2.00 this afternoon so I can't go to the lecture" right?
I am confused about the difference between
"I will see the doctor at 2.00 this afternoon so I can't go to the lecture"
and
"I'm seeing the doctor at 2.00 this afternoon so I can't go to the lecture."

The grammar book says "i'm seeing" is right
but I can't understand why "I will see" is wrong~orz

Question

Is the sentence "I will see the doctor at 2.00 this afternoon so I can't go to the lecture" right?
I am confused about the difference between 
"I will see the doctor at 2.00 this afternoon so I can't go to the lecture" 
and 
"I'm seeing the doctor at 2.00 this afternoon so I can't go to the lecture."

The grammar book says "i'm seeing"  is right 
but I can't understand why "I will see" is wrong~orz

Answer

They are both correct. Sometimes present cont. is used instead of future, and it is more common,  but the future tense is also correct.