Given below are two statements, one labelled as Assertion A and the other labelled as Reason R.Assertion A: Americans withdrew from Vietnam after the Paris Agreement of 1973 on account of political and strategic factors.Reason R: American involvement in Vietnam has been 'machine-intensive'. Added to this was the topographical problems, domestic pressure and public opinion.

• A. Both A and R are true, but R is not the correct explanation of A.
• B. Both A and R are true and R is the correct explanation of A.
• C. A is true, but R is false.
• D. A is false, but R is true.