Which of the following structures contain $sp$-hybridised carbon atom(s)?

I. $\mathrm{HC}\equiv \mathrm{CH}$

II. ${\mathrm{H}}_2\mathrm{C}=\mathrm{C}={\mathrm{CH}}_2$

III. 

IV. 

Arrange the following carbanions in order of their decreasing stability

(i) ${\mathrm{H}}_3\mathrm{C}-\mathrm{C}\equiv {\mathrm{C}}^{-}$

(ii) $\mathrm{H}-\mathrm{C}\equiv {\mathrm{C}}^{-}$

(iii) ${\mathrm{H}}_3\mathrm{C}-{\mathrm{CH}}_2^{-}$

The configuration of the compound