Answer : (A) Both A and R are true and R is the correct explanation of A
Step 1 — Test the Assertion
The two strands of the DNA double helix are indeed complementary — the sequence of one completely determines the sequence of the other. The assertion is true.
Step 2 — Test the Reason
The strands are held together by hydrogen bonds between specific pairs of bases :
$$\text{Adenine} = \text{Thymine} \quad \text{(two hydrogen bonds)}$$
$$\text{Guanine} \equiv \text{Cytosine} \quad \text{(three hydrogen bonds)}$$
The reason is true.
Step 3 — Check whether R explains A
The pairing is not arbitrary — it is forced by geometry and by hydrogen-bond positions :
- A large purine must always pair with a small pyrimidine, so that the double helix keeps a uniform width along its length.
- Only these particular combinations have their hydrogen-bond donors and acceptors correctly aligned to fit together.
Because the pairing is so strictly specified, wherever there is an A on one strand there must be a T opposite it, and wherever there is a G there must be a C. The sequence of one strand therefore fixes the other entirely — which is what "complementary" means.
Step 4 — Note the biological consequence
This complementarity is what makes self-replication possible. The helix unwinds and each strand acts as a template for building its partner, giving two identical daughter molecules.
R therefore correctly explains A.
