1,IS
正确的：the rescuers reached the (skiers and found them apparently unharmed but nevertheless took them) to the hospital for observation
为什么不是(skiers,who were found apparently unharmed,neverthless taking them)
2,IS
Sime scientists believe that mentally stimulating activities may increase the number of brain cells available to replace (any that) are lost through aging.
为什么不是(if any)
3,IE
in 1988 a Soviet icebreaking ship (helped free) three gray whales that (had become trapped) in the Arctic ice after they (had swam) into the costal waters of Alaska (to feed).
为什么had swam错了？