Although it is not officially recorded as extinct, no one __________ (seen) a Yangtze River Dolphin since 2002.
Although it is not officially recorded as extinct, no one HAS SEEN a Yangtze River Dolphin since 2002.
It is believed that Javan Tigers became extinct soon after Caspian Tigers __________ (disappear).
It is believed that Javan Tigers became extinct soon after Caspian Tigers HAD DISAPPEARED.
Scientists confirm that Polar Bears __________ (become) extinct by 2050.
Scientists confirm that Polar Bears WILL HAVE BECOME extinct by 2050.