The "sing-song" theory was put forward by the great Danish linguist ______.

A．If we focus our attention on one thing, we might forget another.
B．Memory depends to a certain extent on the environment.
C．Repetition helps improve our memory.
D．If we keep forgetting things, we'd better return to where we were.

A．they have a wider range of interests
B．they are more reliant on the environment
C．they have an unusual power of focusing their attention
D．they are more interested in what's happening around them