Niels Bohr's model of the hydrogen atom was controversial because .a. it contradicted accepted principles of physics. b. held that electrons existed in allowed orbits and nowhere else. c. accounted for the observed spectrum in hydrogen. d. All of the above.

The correct answer to the question is

a. it contradicted accepted principles of physics

Explanation:

The idea of electrons moving in orbits contradict classical electromagnetism idea whereby electrons are expected spiral to the nucleus instantaneously. Meaning all atoms are supposed to be unstable a postulation that contradicts observable evidence.

Bohr's model of the atom consists of atoms, having electrons in specific orbits. The theory explains spectroscopy and enabled the determination of the constituent atoms present in distant stars.

the awnser is group 12

metric time was an attempt to create a standard unit of time based off of 10, but it never took off an is not included in the metric system (si)