The main problem with early attempts to listen for radio signals from space was
A) that they were conducted at relatively low frequencies which have problems penetrating the Earth's upper atmosphere
B) building receivers sensitive enough to detect extraterrestrial signals
C) that they were conducted at relatively high frequencies which have problems penetrating the Earth's lower atmosphere
D) interference by radio waves from the Sun

Question

Quizplus · Accepted Answer

Early attempts to listen for radio signals from space were conducted at low frequencies, which are absorbed or reflected by the Earth's ionosphere, making it difficult to detect extraterrestrial signals.

The Main Problem with Early Attempts to Listen for Radio