I just took that quiz , it’s A
Doppler effect is the compression or extension of a sound wave, which causes a change in its wavelength / frequency (and so its sound).
Explanation: It is defined as the effect produced by a moving source of waves in which there is an upward shift in frequency for observers, the source is moving towards and downward shift of frequency from which the source is moving away. used to tell if an object in space is moving toward or away from us.
Beryllium, Magnesium, Calcium....etc have two valence electrons
It is a covalent bond. Whenever a compound uses such suffixes like mono, di, tri, tetra, and so on, it is a covalent compound- thus having covalent bonds.
