Вставьте 1.The problem...by Russian scientists (1. has solved, 2. was solving, 3. has been solved) 2. If it... raining we will go for a walk ( 1/stops, 2. will stop, 3. stopped) 3. If I...free, I"ll go to the party (1.was, 2.have been, 3. am.)
1 has been solved 2. stops.3 am