In which of the following conditions would a person not hear an echo?
a. a sound wave reaches the ear, and the reflected wave reaches the ear less than 0.1 seconds later.
b. the reflecting source is approximately 21.5 meters from the person.
c. a person is standing 30 meters in front of the reflecting surface.
d. a person is standing in the range of the sound waves coming from the original sound source as well as the reflected waves.

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.