They can only be considered a stand alone sentence in response to another’s statement or question, or as above.

If your statement, “I think it would be best to study the problem carefully”, is your own to me, then those might be my responses to you unsaid question, “What do I mean?”.

If it precedes the the statements A-D, it is a command for the reader to consider each meanng or intent of the preceding statement, “I think it would be best to study the problem carefully”. “It makes a statement”, etc.

I hope this answer is as confusing to you as your question appears to me.

If your peer, it makes a statement.

If your supervisor, it gives a command

Why should I study the problem carefully?