Concept:Use the sum-to-product identity to combine the sine terms.Formula:sinA+sinB=2sin(2A+B)cos(2A−B)Solution:sinx+sin(x+1)=2sin(22x+1)cos(21)=2sin(x+21)cos21Maximum value of sin(x+21) is 1.Hence maximum value of the sum is 2cos21.Therefore, the constant k is 2.Answer:C. 2