The sum of the coefficients of $$x^{499} \text{ and }x^{500} \text{ in }(1+x)x^{1000}+x(1+x)x^{999}+x^{2}(1+x)x^{998}+....+x^{1000} \text{ is: }$$

A

$$^{1001}C_{501}$$

B

$$^{1002}C_{501}$$

C

$$^{1000}C_{501}$$

D

$$^{1002}C_{500}$$