The first thing I can see in this picture are 2 people
I happened to see this sentence, and it says using 'is' instead of 'are' is wrong.
As I looked up some grammar stuff, you can invert the complement if it is not a noun but an adjective or an adverb.
Also the subject “the first thing” does not seem to be a sort of unit such as 2 inches as in
“2 inches is not that long.”
So I would like to ask why and how the verb should be 'are', not 'is'. Thank you.