Which halogen is known to cause the reaction given below :

$$2 \mathrm{Cu}^{2+}+4 \mathrm{X}^{-} \rightarrow \mathrm{Cu}_{2} \mathrm{X}_{2}(\mathrm{s})+\mathrm{X}_{2}$$

In chromyl chloride, the number of d-electrons present on chromium is same as in

(Given at no. of $$\mathrm{Ti}: 22, \mathrm{~V}: 23, \mathrm{Cr}: 24, \mathrm{Mn}: 25, \mathrm{Fe}: 26$$ )

Element not present in Nessler's reagent is :

Given below are two statements: one is labelled as "Assertion A" and the other is labelled as "Reason R"

Assertion A : In the complex $$\mathrm{Ni}(\mathrm{CO})_{4}$$ and $$\mathrm{Fe}(\mathrm{CO})_{5}$$, the metals have zero oxidation state.

Reason R : Low oxidation states are found when a complex has ligands capable of $$\pi$$-donor character in addition to the $\sigma$-bonding.

In the light of the above statements, choose the most appropriate answer from the options given below