The visitors came too early. When they _____ (come) we _____ (swim) in the pool.

 The visitors came too early. When they have come we swam in the pool.
 The visitors came too early. When they came we were swimming in the pool.
 The visitors came too early. When they were coming we were swimming in the pool.
 The visitors came too early. When they were coming we had been swimming in the pool.