<p>You can be sure without being correct. </p>
<p>See [Short</a> Run and Long Run Supply in Competitive Markets](<a href=“http://instruct1.cit.cornell.edu/courses/econ101-dl/lecture-supply-short&long-run.html]Short”>http://instruct1.cit.cornell.edu/courses/econ101-dl/lecture-supply-short&long-run.html) :
“On the short run supply curve the number of firms in the industry is constant because no firm can change its fixed factors, which include land.”</p>
<p>Also See [Mitchell</a> Langbert’s Blog: Firms’ Goals and Pricing](<a href=“http://mitchell-langbert.blogspot.com/2008/03/firms-goals-and-pricing.html]Mitchell”>Mitchell Langbert's Blog: Firms' Goals and Pricing) :
“In the short run (longer than very short run) the number of firms is fixed but firms can adjust the amount they are producing.”</p>
<p>And with respect to the long run, it makes no sense that they will exit in the long-run. There will be a short-run economic profit, and then they will continue to enter over long-run until a normal (economic profit = 0) is reached. Industries will not leave just because they are making less of an economic profit – the very definition of an economic profit implies that it is not worth leaving!</p>