If the Market Value of an Equity Method Investment Falls

If the market value of an equity method investment falls below its book value:

A) ​it is accounted for at the lower of cost or market so an adjustment is made whenever this occurs.
B) ​it is written down, but when the value rebounds, it is readjusted to its original value.
C) ​it is written down if the decline is considered permanent in nature with no subsequent increase in value.
D) ​it is written down if the decline is considered permanent in nature with no subsequent increase in value other than regular equity method adjustments.