b$.$ Why does the longer$-$term $(15-$year$)$ bond fluctuate more when interest rates change than does the shorter$-$term bond $(1$ year$)?$

I. Longer$-$term bonds have less interest rate risk than shorter$-$term bonds.

II$.$ Longer$-$term bonds have less reinvestment rate risk than shorter$-$term bonds.

III. Longer$-$term bonds have more interest rate risk than shorter$-$term bonds.