|A) Only a follows||B) Both a & b follows|
|C) Only b follows||D) Neither a nor b follows|
The statement mentions that chances of heart ailments are greatly reduced by a regular half-hour exercise. So, a follows. However, it talks of only reducing the probability which does not mean that persons involved in sedentary jobs shall definitely suffer from heart ailments. So, b does not follow.
Hence, concusion a only follows the given statement.