If $\sin^2 A= \sin^2 B+\sin^2 C$, then the triangle is :

$1$. Equilateral

$2$. Isosceles

$3$. Acute angled triangle

$4$. Right angled triangle.

My Attempt:

$$\sin^2 A=\sin^2 B+\sin^2 C$$
$$1-\cos^2 A=1-\cos^2 B+ 1-\cos^2 C$$
$$\cos^2 A -\cos^2 B - \cos^2 C= -1$$

How do I proceed further?