This is a test of commonsense with single/multi-choices. Complete the next sentence:

How to bodyboard
Put safety first.
If you want to be able to body board, then you should already be a good swimmer. You'll need to use a lot of the techniques you use for swimming to propel your board, and furthermore, you'll need to be a good swimmer to swim without your board if you wipe out.

OPT:
a). One good idea is to learn a few skills about basic commands, then work your way up to professional swimming skills in sports. This will also make you a more interesting skier.
b). You should also aim for doing low rope endurance exercises in water frequently. This is because when you are in the water, your body comes into contact with the force of the waves.
c). For proper form, you should hit your knees with long, downward sweeping strokes, and be sure to avoid your feet hitting the ground. Position yourself high in your board, below your board so that you have balance and momentum.
d). Additionally, you should only try bodyboarding if you know that the sea conditions are safe and that there's a lifeguard on duty. You should try bodyboarding with a friend or an instructor instead of by yourself.
The answer is:
d).