Which set of observations would most strongly suggest a possible chemical attack rather than a routine incident?

Explanation

Chemical-attack indicators taught in CDC and DHS modules include sudden clustered casualties with similar symptoms, unusual odors (almonds may suggest cyanide, garlic may suggest mustard agent, freshly cut grass may suggest phosgene — although many agents are odorless), dead vegetation or animals in proximity, and witnesses reporting eye/skin/respiratory irritation. Single illness (a) is generally not a chemical attack signal; routine deliveries (c) and normal HVAC (d) do not suggest attack. Response: move upwind/uphill, deny entry, call 911 specifying possible chemical incident.