Answer :
Answer: D
Explanation: They can be sure if they both take a blood test. There is a possibility both jane and john are carriers of sickle cell(AS), which makes it possible to have children with sickle cell disease. Also, both jane and john may not be carriers at all (AA), in that case none of their child will have sickle cell, likewise if either of them is AA, and the other is a carrier, none of their child will have sickle cell.