Jennifer made an error converting y=x^2+2x+7 to vertex form. In which step did she make the error? Explain your answer and provide the correct form of the equation in vertex form.

A. y=x^2+2x+7
B. y=(x^2+2x)+7
C. y=(x^2+2x+1)+7
D. y=(x+1)^2+7