JEE Math Practice Question

If the curves, $${{{x^2}} \over a} + {{{y^2}} \over b} = 1$$ and $${{{x^2}} \over c} + {{{y^2}} \over d} = 1$$ intersect each other at an angle of 90$$^\circ$$, then which of the following relations is TRUE?

1. A.a $$-$$ c = b + d
2. B.a + b = c + d
3. C.$$ab = {{c + d} \over {a + b}}$$
4. D.a $$-$$ b = c $$-$$ d