I`ll start it for you :
１.Girls are happier in the country than ＿＿＿in the city.
A. that B. it C. / D. those, meaning:
Girls are happier in the country than the girls [= those]in the city.
2. the same
3.The school is more beautiful now than____ in the past.
A. that B. it C. / D. one, meaning :
The school is more beautiful now than the school [= that] in the past.
Does that help?