In addition to the standard ring systems (such as cyclohexane), cyclic compounds can also be bicyclic, tricyclic, etc., or they can be spirocyclic, bicyclic or bridge head carbons. The point of attachment of two rings are called bridge head carbons. The point of attachment of two rings are called bridge head atoms Some bicyclic compounds like camphor are commonly found in plants. Others like norbornane can be synthesized in the laboratory. The formal names of of bicyclic and related ring systems are based on : (1) Total number of atoms in the molecule. (2) The number of atoms in each bridge connecting the bridge head atoms. These numbers are written in square bracket in decreasing order. Spirocyclic compounds have two fused rings, but only one bridge head atom. Spirocyclic compounds are named like bicyclic compounds, but have the prefix spirocyclo. Answer the following questions : Which of the following is the correct structure of bicyclo [1.1.0] butane ?