---Tom, why not stop ________ a rest?

--- Oh, no, Mum. It’s so exciting that I can’t stop _______ the kite.

A．having; flying

B．to have; to fly

C．having; to fly

D．to have; flying

下列哪项是在细胞工程中细胞改造的良好材料（）.

A.真核细胞

B.原核细胞

C.体细胞

D.病毒