CREATE TABLE table_17973 ("Round" FLOAT, "Circuit" CLOB, "Date" CLOB, "Length" CLOB, "Pole Position" CLOB, "GT3 Winner" CLOB, "GTC Winner" CLOB)

what are all the gtc winners for pole position no. 21 team modena
SELECT "GTC Winner" FROM table_17973 WHERE "Pole Position" = 'No. 21 Team Modena'
SELECT "GTC Winner" FROM "table_17973" WHERE "Pole Position" = 'No. 21 Team Modena'