Which of the Following Best Explains the Growth of Telephone

Which of the following best explains the growth of telephone and online selling?

A) the use of the Do Not Call Registry
B) the ability to make up to 8 times as many more customer contacts per day than an outside salesperson
C) the similarity in cost between the inside and outside sales forces
D) the difficulty and expense of reaching some customers
E) the preference of customers for face-to-face contact