Mir Qasim became the Nawab of Bengal with the help of the English, but why was Mir Jafar reinstated later on?

Consider the following statements:
Assertion (A): During the reign of Siraj-ud-Daulah of Bengal, the English East India Company began to fortify Calcutta without the Nawab's permission.

Reason (R): English Company had already got permission from Emperor Shah Alam II to fortify Calcutta in anticipation of their fight with the French stationed at Chandernagore.

$1$. The Battle of Plassey was won by the British more by their diplomatic skill than by their strength of arms.
$2$. The Battle of Buxar has culminated with the Treaty of Allahabad and settlement with the Nawab of Awadh.
Select the correct answer using the codes given below.