Natalie Sent an E-Mail to Portman Asking Him to Review

Natalie sent an e-mail to Portman asking him to review a file, and asked Portman to acknowledge receipt of the e-mail. Portman thought that a response was not necessary and started reviewing the file without replying to Natalie. Five hours later, a frustrated Natalie sent a second e-mail to Portman asking whether he had received her initial message. What hindered the communication process in this scenario?

A) Lack of feedback
B) Inadequate information
C) Information overload
D) Problems with channels