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

  the   balanced   equation   for   reaction of   ca metal   with   o2 to   produce   cao   is as below

2 ca+ o2→ 2 cao2

  explanation

the reaction is   balanced     since the   number of atoms   in reactant side is   equal to   number of atoms   in   product side.   for   example   they   are 2   atom of   ca in   reactant   side   and   2   atoms of ca   in   the product side.

  the   process   is a   meet the   requirement   of   redox   reaction because :     calcium   is   oxidized   from   oxidation state 0   to 2+   while   oxygen   is reduced from   oxidation   state   0   to 2-

the answer is option a.