aidez moi svp je ne comprend pas cedt urgent je vous remercie

salut ! 
ex 3 :
a) I was reading an exciting story when mum asked me to set the table.
b) I was walking the dog in the park when my mobile rang.
c) I was painting the garage door when my grandmother sent me to the supermarket.
d) I was brushing my teeth when my little sister jumped on me.