An ideal gas is made to undergo a cycle depicted by the $p-V$ diagram given below. The curved line from $A$ to $B$ is an adiabat. Then,

• A
  the efficiency of this cycle is given by unity as no heat is released during the cycle
• B
  heat is absorbed in the upper part of the straight line path and released in the lower part
• C
  if $T_1$ and $T_2$ are the maximum and minimum temperatures reached during the cycle,then the efficiency is given by $1-\frac{T_2}{T_1}$
• D
  the cycle can only be carried out in the reverse of the direction shown in the above figure