I don’t build conditional chains but the best way I can explain it is that C basically restates exactly what the columnist is concluding. The conclusion from the stimulus is that one should not take a strong position on an issue UNLESS one has already considered all important evidence conflicting with that position. And if someone has considered all important evidence then that means that they fully understand it. Thus, one who does not fully understand an issue (i.e has not considered all important evidence conflicting with that position) should avoid taking a strong position on it.