The correct answer to this question is Water. There are many times when plants face dry conditions. Water is crucial for plants to grow and during a drought, there is little to no water. Without water, a plant's ability to survive is hindered. Being that they face dry conditions often, the plant must adapt to this environmental change.
There are a number of ways they do this, which allow some plants to continue on for a short amount of time without water. Doing these techniques allow a plant to combat the shortage of water they experience and protect them from toxic chemicals.