Help. Which one is the appropriate meaning of infinitive marker "to" in the sentence?
The sentence: It's plain to see

I know it means very clear, but don't know the meaning of infinitive marker "to".

■expressing purpose or intention.
I set out to buy food.

■expressing an outcome, result, or consequence.
he managed to escape.

■expressing a cause.
I'm sorry to hear that.

■indicating a desired or advisable action.
we asked her to explain.The sentence: It's plain to see

I know the sentence means it's very clear.

Question

Help. Which one is the appropriate meaning of infinitive marker "to" in the sentence?
The sentence: It's plain to see

I know it means very clear, but don't know the meaning of infinitive marker "to".

■expressing purpose or intention. 
  I set out to buy food.

■expressing an outcome, result, or consequence. 
  he managed to escape.

■expressing a cause. 
  I'm sorry to hear that.

■indicating a desired or advisable action. 
  we asked her to explain.The sentence: It's plain to see 

I know the sentence means it's very clear.

Answer

talkie,

It's plain to see.

It.....refers to something already mentioned or known  by context.

is......a linking verb

plain....a predicate adjective meaning "clear" or "easy" modifying the subject.

to see.....an infinite functioning as an adverb modifying the adjective by indicating the purpose to which the predicate adjective "plain" refers. 
"To see" in this sentence can mean to perceive by the eye, to understand, or to visualize.

It is plain for the purpose of seeing.

Answer

It is plain to see.
"It" is the subject/noun
"is" is a BEING VERB. This verb does not show action, but tells about something in a state of being.  Sometime, it just is.  For that purpose, you use a being verb, "to be" = "is".
"plain to see" is a prepositional phrase that acts like an adjective, describing "it".

Answer

I seriously didnt understand your question.

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