Phosphorus is kept in water because A. its ignition temperature is very high B. its ignition temperature is very low C. its critical temperature is high D. its critical temperature is low

The correct answer is: A. its ignition temperature is very high.

Phosphorus is a highly reactive element that can ignite spontaneously in air at temperatures as low as 40 degrees Celsius. It is therefore kept in water to prevent it from igniting. The ignition temperature of water is much higher than that of phosphorus, so it will not ignite while it is submerged in water.

Option B is incorrect because the ignition temperature of phosphorus is actually very low. Option C is incorrect because the critical temperature of phosphorus is not particularly high. Option D is incorrect because the critical temperature of water is much higher than that of phosphorus.