Which of the following is a typical symptom of a "sweller" can?

A typical symptom of a "sweller" can is that both ends bulge outward. This bulging occurs due to the buildup of gases inside the can caused by microbial action or spoilage, which increases the internal pressure. As pressure builds, it can cause the can to deform, resulting in both ends of the can bulging outward rather than remaining flat. This is a clear indicator that the contents are compromised and potentially unsafe for consumption.

The other conditions mentioned, such as one side being concave or the presence of rust spots, do not accurately define a "sweller" can. A concave appearance would indicate external pressure or a different type of damage, while minor rust spots may indicate corrosion but do not directly relate to the swelling phenomenon typical of bacterial or yeast action inside a can. Similarly, an intact surface with no leak doesn’t signify swelling; rather, it implies that the can may still be sealed but is not a clear indicator of its internal condition.