Why Was the Discovery of Pulsars Initially Misinterpreted as Evidence

Why was the discovery of pulsars initially misinterpreted as evidence of intelligent life elsewhere in the universe?

A) It was not thought possible for a "natural" radio source to produce the rapid and extremely regular radio pulses detected from space.
B) Radio telescopes occasionally detected the same sequence of five musical notes, which were then whistled or hummed regularly by all the people associated with these telescopes (D, E, C, lower C, and G on a piano) .
C) Pulses arriving from several nearby star systems showed Doppler shifts of frequency apparently caused by orbital motion of their source around the central stars, as if coming from planets.
D) The rate of pulses detected from space appeared to contain primitive coding similar to a crude Morse code.