"Although it is convenient to live in the city, it is also more expensive."

Translation: 虽然住在城市比较方便，但是也比较贵。

[PeaceJoyPancakes]

As it currently stands, the English doesn't say that it's more convenient to live in the city, only that it's convenient. It should therefore be acceptable to leave out the first 比较.

[ArtBurnap]

Chinese predicate adjectives often require some kind of modifier, such as 很, 太, or 比较. Haven't you been marked wrong yet anywhere else because you forgot to put one in?