We were swimming in the lake _____ suddenly the storm started.

We were swimming in the lake _____ suddenly the storm started.

We were swimming in the lake ________ suddenly the storm started.

1. A.
   when
2. B.
   while
3. C.
   until
4. D.
   before

We were swimming in the lake _____ suddenly the storm started.

A．when             B．while             C．until              D．before