never before have i seen such a tower that is being constructed in thr centre of the city
定语从句,先行词是tower
在从句中作主语【the tower is being constructed in the centre of the city】
所以用that为什么不用which只能用that因为有such...that...是固定搭配额。。这是单选题。。只能用that因为有such...that...是固定搭配