Answer:
I know a doctor whose last name is Doctodoctor.
If it's "The last names Doctodoctor" then it sounds like you're talking about your last name.
The answer is B because "I have visited many cities including the following" indicates that there is going to be a list of cities. A colon is used to introduce a series of items or to separate two independent clauses; however the second sentence has to add on to the first.
The answer in my opinion is A. It would really show how big the Great Wall of China is and prove it is one of the largest building projects.
Hope this helped ☺️
A bag is the indefinite article so i would guess c
