An allotrope of carbon which exhibits only two types of $C-C$ bond distances of $143.5 \text{ pm}$ and $138.3 \text{ pm}$, is $....$

The correct order of catenation is

The element that does not show catenation is

Which of the following species is not a pseudohalide?

Sulphur is most soluble in

$A$ red solid is insoluble in water,but it becomes soluble if a small amount of $KI$ is added to the water. If this red solid is heated in a test tube,it produces a violet-colored vapor and some metallic droplets are observed on the cooler parts of the test tube. This red solid is........