What's this? It's an ________.

1. A.
   ruler
2. B.
   clock
3. C.
   pear
4. D.
   apple

What's this? It's an ________.

[　　]

--- What’s this?
--- It’s             .

—What’s this?
—It’s __________ UFO.
A．a                  B．an                  C．the

What’s this?
It’s        “Nn” and       “u”.