About a question in 'Longman preparation course for the TOEFL test'

Hi everyone !
I am preparing for TOEFL test and starting with “Longman preparation course for the toefl test” writen by Deborah Philips
I am confused by a question in it and need help :cry:

Choose the best answer to fill in the space
" _____ appears considerably larger at the horizon than it does overhead is merely an optical illusion "
(A) The Moon
(B) That the Moon
(C) When the Moon
(D) The Moon which

I chose the answer (A) The Moon but the real answer is (B) That the Moon but i Don’t know the reason for it .I’m wondering whether there is any special usage of “that” here ?
Please help me .
Thanks in advance

P/s : And if someone also use it , we can be together to discuss about it . hah ?! :wink:

TOEFL listening discussions: A conversation between a university student and a professor in the professor’s office

Hi Giang93

Using “that” the way it is used in your sentence is similar to using “the fact that”.
Perhaps looking at your sentence as two separate sentences will help:

[color=blue]1. The moon appears considerably larger at the horizon than it does overhead.
[color=red]2. That is merely an optical illusion.

The word “that” in sentence 2 refers to all of sentence 1. The word “that” refers to the fact or idea that is stated in sentence 1. You can combine those sentences beginning with the word “that”.

“[color=red]That [color=blue]the moon appears considerably larger at the horizon than it does overhead [color=red]is merely an optical illusion.”
.