✨ ベストアンサー ✨
思った
..., because I thought this university was at a level of difficulty that could challenge me.
..., because I thought this university was at a level of difficulty at which I could challenge myself.
思う
..., because I think this university is at a level of difficulty that can challenge me.
..., because I think this university is at a level of difficulty at which I can challenge myself.