You mean Tungsha Islands? I the drills are targeted against the Itu Aba Island (Taiping Island). By the way, for your information, the name Itu Aba was officially given to the island by the Republic of China government in 1935. The name "Pratas Island" is not recognized as a official name by the PRC, nor the ROC, even today.IMO they'll invade the Pratas island- it's most distant from Taiwan & has strategic location in the North SC Sea.
It also has a long airstrip & good lagoon: