D为什么是模型的假设？体现在哪里？
问题如下图：
选项：
A. 
B. 
C. 
D. 
解释：
NO.PZ2016082405000011 All of the following are assumptions of the Merton mol except: the risk-free rate is perfectly collinewith the value of the firm. fault conly occur when the bonmatures. bonanstockholrs cannot negotiate. there is no neeto aust for liquity. A The Merton mol assumes ththe risk-free interest rate is constant through time. A还是没太明白，能不能一下呢？
NO.PZ2016082405000011 All of the following are assumptions of the Merton mol except: the risk-free rate is perfectly collinewith the value of the firm. fault conly occur when the bonmatures. bonanstockholrs cannot negotiate. there is no neeto aust for liquity. A The Merton mol assumes ththe risk-free interest rate is constant through time. 请问，C是什么意思？怎么理解？
All of the following are assumptions of the Merton mol except: the risk-free rate is perfectly collinewith the value of the firm. fault conly occur when the bonmatures. bonanstockholrs cannot negotiate. there is no neeto aust for liquity. A The Merton mol assumes ththe risk-free interest rate is constant through time. 请问老师什么不对？哪里假设了不需要调整流动性呢？ 能否把每个都讲解一下呀，谢谢！
跟讲义提到的假设怎么对应起来？
