Determine the correctness or otherwise of the following Assertion (A) and the Reason (R). Assertion : N-methyl-N -nitro-N-nitrosoguanidine (NTG) is an effective chemical mutagen. Reason : Mutations induced by NTG mainly are the GC AT transitions.

• A. both (A) and (R) are true and (R) is the correct reason for (A)
• B. both (A) and (R) are true but (R) is not the correct reason for (A)
• C. (A) is true but (R) is false
• D. (A) is false but (R) is true