AHSME 1974 Problem 3

The coefficient of $x^7$ in the polynomial expansion of $$(1+2x-x^2)^4$$ is $\mathrm{(A)\ } -8 \mathrm{(B) \ }12 \mathrm{(C) \ } 6 \mathrm{(D) \ } -12 \mathrm{(E) \ }\text{none of these}$