select the correct state of hybridization at $$X_1, X_2, X_3 and X_4$$
$$X_1$$ $$X_2$$ $$X_3$$ $$X_4$$
(a) $$sp^2$$ $$sp$$ $$sp^2$$ $$sp^3$$
(b) $$sp^3$$ $$sp^2$$ $$sp$$ $$sp$$
(c) $$sp^2$$ $$sp^2$$ $$sp$$ $$sp^3$$
(d) $$sp^3$$ $$sp^2$$ $$sp^2$$ $$sp$$