The graph is missing, however we can answer anyway.
In fact, this is the approximate percentage of the elements listed in the questions in the Earth's crust (data taken from wikipedia): Iron 5.0 % Calcium 3.6 % Sodium 2.8 % Potassium 2.6 % Magnesium 1.5 %
of all of these, we see that the combination of two elements that makes up the closest number to 7% is calcium and sodium (3.6% + 2.8%=6.4%), therefore the correct answer is Calcium and sodium.
The channel is whatever was used to carry information from one place to another, or from one person to another. In this scenario, the channel is e-mail.
