Lead nitrate on heating gives

When crystals of lead nitrate are heated strongly in a dry test tube :

When we heat lead nitrate [P(NO₃)₂] in a boiling tube, we observe the emission of brown fumes. Which one of the following is the brown gas?

Which one of the following nitrogen oxides may dimerize?

A liquid is heated up to a certain temperature. Which one of the following situation would correspond to the boiling of the liquid?

Which of the following slows down the reaction rate?