When a school bus is stopped on the roadway, you must always stop when

Each driver must stop when a school bus has its Stop sign extended and its red lights are flashing.

Explanation:

When driving behind a school bus, it is best to keep a greater distance than would be taken behind a regular vehicle, as it is highly likely that you are picking up or dropping off students. This gives you more time to stop when the yellow light flashes and the STOP signal comes on.

answer: atomic mass was originally used to arrange elements on the periodic table, but is no longer used to organize the modern version.

explanation:

originally atomic mass was used to arrange elements on the periodic table from left to right in a row. each column then shows similar properties of elements of a group.

in modern periodic table, the atoms are arranged in increasing atomic number. as atomic number represents the number of protons in an atom which is unique for every atom.