Confusing answer to CSS grid quiz

Hi, just doing the second quiz in CSS grid course and this came up…

Can someone help me understand the answer? As far as I can see, if there are 2 rows and 2 columns, that makes 4 grid cells that are all occupied by the 4 boxes, so there won’t be any explicit cells to fill and the fifth box will be implicit and so 70px wide. What am I missing?

Many thanks, Bils

I figured it out, so I’ll post here in case anyone else has the same trouble I did (please let me know if my explanation is wrong so I can correct it). The new cells will be rows and will inherit the existing column width because that is the default, for new columns to be produced, grid-auto-flow: column; would have to be stated… I figured it out with this…