Is 9999b.c in the 90th century bc?

No, 9999 B.C. is not in the 90th century B.C. The century system counts backward in increments of 100 years, so the 90th century B.C. would span from 9000 B.C. to 8901 B.C. Therefore, 9999 B.C. falls in the 10th century B.C., which ranges from 1000 B.C. to 901 B.C.