麻烦解释一下啊

问题如下：

Banks are required to maintain 8 percent of their assets as "Tier 1 Capital". Which of the following count towards this capital requirement?

I. Shareholders equity.

II. Sovereign debt held in the trading book.

III. Common stock of other banks.

IV. Subordinated debt issued by the bank in question (subject to certain qualifying rules).

选项：

A.

I, II, and IV

B.

II and III

C.

I and IV

D.

I only

解释：

Only equity capital and disclosed rescues (primarily after the retained earnings) qualify as Tier 1 capital.