Among the following, the energy of 2s orbital is lowest in :

The IUPAC name for the following compound is :

An 'Assertion' and a 'Reason' are given below. Choose the correct answer from the following options :
Assertion (A) : Vinyl halides do not undergo nucleophilic substitution easily.
Reason (R) : Even though the intermediate carbocation is stabilized by loosely held p-electrons, the cleavage is difficult because of strong bonding.

In the following skew conformation of ethane, H'–C–C–H'' dihedral angle is :