老师您好，想请老师讲解一下下面真题的三个小问：
具体科目：execution：
2008 Q8 B，DPF股票中关于BID/ASK spread的“a large volume。。。”那句话不是很能和这个spread关联，请讲解一下；
2012 Q3 B ，为什么implementation shortfall “attempts to minimize the weighted average of market impact and the opportunity costs of missed or delayed trades.”？“To minimize these opportunity costs, implementation shortfall will “front-load” trade execution to complete the trade more quickly than either TWAP or VWAP.”又是指？
2012 Q3 c 最后一点最后一句话“This is consistent with the client’s willingness to accept additional risk as his portfolio value increases”，同样满足这个条件的还有buy and hold吧？那么这个理由如果不具有唯一性的话，放在这里合适吗？
