My answer is none of those four, and I'll make the case with one number.

September, measured two ways

Take the S&P 500 as it's normally quoted, weighted by market capitalisation. It fell 0.58% last month. A nothing month, the sort you'd forget about.

Now take the exact same five hundred companies and weight them equally, so Apple counts the same as the four hundredth name on the list. That version fell 5.18%.

Same businesses. Same month. A gap of more than four and a half percentage points.

The rest of the board agrees with the second number rather than the first — the Dow fell 4.33%, and the Russell 2000 fell 5.46%. The only thing that went up was the Nasdaq 100, at +3.21%, and that is the most concentrated index of the lot.

So September wasn't a flat month that happened to have some weak sectors.