In order to guarantee option contract performance, the Option Clearing Corporation $($OCC$)\_\_\_\_\_\_\_\_\_\_\_\_\_\_.$

a$.$ requires option holders, but not option writers, to post margin

b$.$ allows writers of put options to satisfy margin requirements by holding the underlying stock in a brokerage account

c$.$ requires increased margin from the writer of in$-$the$-$money options compared to out$-$of$-$ the$-$money options

d$.$ all of the above statements are true about option margin requirements