Project: selector
Task (challenging): modify html and css for `#table`:
- `cell(x,y)` means the yth cell in the xth row
- append `<p>` in 4 cell(1,2), cell(2,3), cell(3,4), cell(4,2)
- set `background-color` of cells with `<p>` to `yellow`
- do not use `!important` anytime