Hallo is how you say hi in german
a. Wie geht’s dem Lehrer? Ihm geht’s gut.
b. Wie geht’s den Kindern? Ihnen geht’s prima.
c. Wie geht’s den Großeltern? Ihnen geht’s schlecht.
Note all options are used here. but based on genus, casus and numerus, that's the only correct version I can see.
reasoning: native speaker
The measure of an arc is equal to the measure of an arc's central angle.
So the measure of an arc with a central angle of 90 ° is also 90 °.
