90. Our nanny used to tell us bedtime stories. -

Our nanny used to tell us bedtime stories. -  б) so my mum did.
Because she did it often or usually for a long time and nanny did it also for a long time. 
a) isnt correct because mother didnt tell storries for one time. 
в) uncorrect sentence
д) uncorrect sentence
г) Mom told stories, and the sentence says that she did not do this.