With no explanation, label text_A→text_B with either "DON'T KNOW", "NO" or "YES".
text_A: The historical calving grounds were unknown but might have been along southern Chinese coasts from Zhejiang and Fujian Province to Guangdong, especially south of Hailing Island and to near Hong Kong. Possibilities include Daya Bay, Wailou Harbour on Leizhou Peninsula, and possibly as far south as Hainan Province and Guangxi, particularly around Hainan Island. These areas are at the southwestern end of the known range. While the full extent of the whales' normal range in earlier times are unknown, there is evidence of the mammals regularly inhabiting the Gulf of Tonkin. In addition, the existence of historical calving ground on Taiwan and Penghu Islands (with some fossil records and captures), and any presence in other areas outside of the known ranges off Babuyan Islands in Philippines and coastal Vietnamese waters beyond the Gulf of Tonkin are unknown. There is only one confirmed record of accidentally killing of the species in Vietnam, at Ngoc Vung Island off Ha Long Bay in 1994 and the skeleton is on exhibition at the Quang Ninh Provincial Historical Museum. Gray whales are known to occur in Taiwan Strait even in recent years.
text_B: If scientists were to embark on a systematic search for historical evidence of these whales, to try to find the southern edge of the region in which we can know that the whales once swam, would their logical starting point be on a lattitude that crosses through, or possibly even north of, the Gulf of Tonkin?
NO.