Gravitational waves were first predicted by Einstein in 1916. The first direct detection however, from an Earth-based instrument, was only made possible a century later, in 2015, by the Laser Interferometer Gravitational-Wave Observatory (LIGO). Select the reasons why it is difficult to detect gravitational waves on Earth. Gravitational wave sources are rare. Motions such as earthquakes can mimic the effect of gravitational waves. Earth's magnetic field alters gravitational waves. The detection of gravitational waves require measurements in near-perfect vacuum. Gravitational waves are very weak once they reach Earth. Earth's atmosphere absorbs some part of the signal