Among following compounds, the number of those present in copper matte is ___________.

A. $$\mathrm{CuCO_{3}}$$

B. $$\mathrm{Cu_{2}S}$$

C. $$\mathrm{Cu_{2}O}$$

D. $$\mathrm{FeO}$$

The number of electrons involved in the reduction of permanganate to manganese dioxide in acidic medium is _____________.

How many of the following metal ions have similar value of spin only magnetic moment in gaseous state? ______________

(Given : Atomic number V, 23; Cr, 24; Fe, 26; Ni, 28)

V$$^{3+}$$, Cr$$^{3+}$$, Fe$$^{2+}$$, Ni$$^{3+}$$