Step 1: Scurvy is caused by a deficiency of Vitamin C (ascorbic acid), with symptoms such as bleeding gums and slow healing of wounds. So A is true. Step 2: Lemon, orange, amla and guava are rich in Vitamin C. So R is true. Step 3: The reason explains the assertion: because citrus fruits supply Vitamin C, they prevent the deficiency disease scurvy. Answer: Both A and R are true, and R explains A.