You got it.

And you got the resistance vs impedance. Although we use ohms to quantify both, resistance implies that the voltage and current are always in phase (0 degrees, like for a resistor) while impedance implies that they are out of phase. When you plot impedance then on an XY graph, it'll have a "real" part and an "imaginary" part (implying phase). The magnitude of that vector is measured in ohms.

Since you're in class, another way to think of the lower speaker impedance being more difficult to drive is to consider power. P=V^2/R. So if we assume that the voltage is the same (volume knob in the same spot), when the "R" gets smaller, the power goes up.