关于物质的密度，下列说法中正确的是（　　）

A．密度是表示不同物质的物体的质量不同

B．密度是表示同种物质体积相同时，其质量不同

C．密度是表示同种物质质量相同时，其体积不同

D．密度是表示不同物质体积相同时，其质量不同

Vega research has been conducting investor polls for Third State Bank. They have found the most investors are not willing to tie up their money in a 1-year (2-year) CD unless they receive at least 1.0% (1.5%) more than they would on an ordinary savings account. If the savings account rate is 3%, and the bank wants to raise funds with 2-year CDs, the yield must be at least:（）

A. 4.0%, and this represents a required rate of return.

B. 4.5%, and this represents a discount rate.

C. 4.5%, and this represents a required rate of return.