A7. A straight angle is divided into four angles by three rays, the sizes of which are in the ratio $1: 2: 5: 1$. What is the measure of the largest angle?
(A) $20^{\circ}$
(B) $50^{\circ}$
(C) $100^{\circ}$
(D) $200^{\circ}$
(E) $270^{\circ}$
A7. The sizes of the angles, which in the ratio $1: 2: 5: 1$ form a straight angle, are $1 x, 2 x$, $5 x$, and $1 x$. It holds that: $1 x+2 x+5 x+1 x=180^{\circ}$ or $x=20^{\circ}$. The largest angle measures $5 x=100^{\circ}$.