You are given several rational expressions that involve perfect-square trinomials. For each expression below, simplify by factoring and dividing; state any restrictions on the variables so no denominator is zero.

a. Simplify: [math]\dfrac{x^2 - 4x + 4}{b^2 - 4b + 4} \div \dfrac{x - 2}{b - 2}

b. Simplify: [math]\dfrac{x^2 - 6x + 9}{c^2 - 6c + 9} \div \dfrac{x - 3}{c - 3}

c. Simplify: [math]\dfrac{x^2 - 8x + 16}{d^2 - 8d + 16} \div \dfrac{x - 4}{d - 4}

d. Simplify (general case): [math]\dfrac{x^2 - 2kx + k^2}{m^2 - 2km + k^2} \div \dfrac{x - k}{m - k} \quad(\text{with parameters }k,m)