Sal had a severe stroke that left him unable to talk for a year. however, during the second year, he gradually regained his ability to speak. what is the most probable explanation for what happened?

Neurogenesis

Explanation: after a stroke, the brain has the ability to heal itself.

In a stroke, a blood vessel is blocked or has a rupture, so the blood can't reach some areas of the brain. Since the neurons don't have blood, they die and this part of the brain, and also the nerve cells, get damaged.

However, the brain can regenerate these brain cells. It is a process called Neurogenesis, and it is basically a process of creating new connections. This process can months or years, depending on the severity of the damage.  

question - in 1827, john walker, a druggist in a small english town, tipped a splint with sulphur, chlorate of potash, and sulphid of antimony, and rubbed it on sandpaper, and it burst into flame. the druggist had discovered the first friction-chemical match, the kind we use to-day. it is called friction-chemical because it is made by mixing certain chemicals together and rubbing them. although walker's match did not require the bottle of acid, nevertheless it was not a good one. it could be lighted only by hard rubbing, and it sputtered and threw fire in all directions. in a few years, however, phosphorus was substituted on the tip for antimony, and the change worked wonders. the match could now be lighted with very little rubbing, and it was no longer necessary to have sandpaper upon which to rub it. it would ignite when rubbed on any dry surface, and there was no longer any sputtering. this was the phosphorus match, the match with which we are so familiar. according the paragraph what was the benefit of the new phosphorus match? it burst into flame when rubbed on sandpaper. it was made by mixing chemicals. it could be ignited on any dry surface. it threw fire in all directions.

answer - matches required natural chemicals