This kind of exercise is most _____, and therefore is a good test of you students' ability. [ ]

A. surprising

B. encouraging

C. exciting

D. challenging

在证券经纪业务中，客户应与证券经纪商之间建立特定的经纪关系，这一关系的建立过程包括签订证券交易委托代理协议和开立资金账户。( )