问题如下：
Which of the following equity securities that an investor consider the least risky?
选项：
A.Putable preference shares.
B.Callable preference shares.
C.Non-callable preference shares.
解释：
A is correct.
Putable shares, whether common or preferred, can be sold back to the issuer at a predetermined price.
The predetermined price creates a floor for share prices, reducing uncertainty about future cash flows for investors.
请问下为啥 C 会比 A 风险大呢？是因为 A 可以按照约定的价格卖给公司，如果跌到一定程度了，实在不行还有个 floor price，而 C 的话最低可以跌到 0，于是风险就比 A 大了，是这样么？
