―Tom, you look puzzled. Do you understand it?

―Yes. I __________ another problem just now.

      A. have thought of                               B. had thought of

      C. was thinking of                               D. am thinking of

—Tom, you look puzzled. Do you understand it?

—Yes. I ________ another problem just now.

[　　]

—Tom, you look puzzled．Do you understand it?

　 —Yes．I __________ another problem just now．

A．have thought of　　　　　　　　 　　 B．had thought of

C．was thinking of　　　　　　　　　　 D．am thinking of