Which of these is a property of ice that allows fish to survive in ponds when air temperatures are below $0^{\circ}$ C?

Question

$\begin{array}{1 1} \text{a. Ice floats, insulating warmer liquid water below it.} \\ \text{ b. Ice sinks, leaving a large body of liquid water in which fish can move.} \\ \text{ c. Ice slows fish metabolism until warm spring temperatures increase them.} \\ \text{ d. Ice allows oxygen to enter the water when temperatures are below freezing.} \end{array} $