เมืองใดมีลูกค้ามากที่สุด?
Which city does has most number of customers?
df_customers = pd.DataFrame(columns=['customer_address_id'])
df_addresses = pd.DataFrame(columns=['city', 'address_id'])
df_addresses[df_addresses['address_id'].isin(df_customers['customer_address_id'])]['city'].value_counts().idxmax()