Which of the following is a valid statement related to accounting for goodwill per ASU $2017-04?$

a$.$ Implied fair value of goodwill must be calculated to record an impaiment loss.

b$.$ If it is more likely than not that goodwill is impaired, then fair value of the reporting unit assigned the goodwill must be determined.

c$.$ Negative goodwill may result from recording of an impairment loss.

d$.$ Nonpublic entities may no longer elect to amortize goodwill under the Private Company Council alternative.