擦，又一道数学题不会做了，求助。

Let R be a ring with a multiplicative identity. If U is an additive subgroup of R such that ur 2 U for all
u 2 U and for all r 2 R, then U is said to be a right ideal of R. If R has exactly two right ideals, which of
the following must be true?
I. R is commutative.
II. R is a division ring (that is, all elements except the additive identity have multiplicative inverses).
III. R is infinite.
(a) I only (b) II only (c) III only (d) I and II only (e) I, II, and III
{:7_182:}