Read the following paragraph. which sentence is the topic sentence?

i first stopped by the store to pick up a few groceries. then, i dropped off my letters at the post office. i had a dentist appointment in the afternoon. finally, i attended my best friend’s birthday party in the evening. my day was long and exhausting.
a)i first stopped by the store to pick up a few groceries.
b) then, i dropped off my letters at the post office.
c) i had a dentist appointment in the afternoon.
d) finally, i attended my best friend's birthday party in the evening.
e)my day was long and exhausting.