Which word in the passage can replace the expression 'unpredictable behaviour’?

Directions: Read the passage and answer the given question.

These two factors, the altitude and the weather, tend separately and together to defeat the climber. The height weakness slows him down. It forces him to ­spend days and nights in the course of his assault on the summit. The weather, besides adding to the ­demands on his energy and moral fortitude, conspires to deny him the time he needs to complete his mission. Whereas in lower mountains and on easy ground the weather may be no more than a handicap, in the high Himalayas it is decisive, regardless of terrain.

The deduction to be drawn from these two factors was clear enough. We must either fortify ourselves so that we could continue, without detriment, to live and have our being above the limit of natural acclimatisation, or, better still, we must solve the problem of speed. It was desirable, in fact, that we should meet both these requirements and thus give to those chosen to attempt the summit and to their supporting teams some measure of insurance against ­the vagaries of the weather, for safety in mountain climbing is as much a matter of swiftness as of sureness of foot. Either or both could be achieved only by the administration of oxygen in sufficient quantities to make up for the deficiency in the air, and for the duration of the upward journey above the limit of successful acclimatization.