It is grammatically correct to spell out the word for numbers of ten and less, hence, "second century B.C." is correct.
BC (Before Christ) AD (Anno Domini)......meaning "In the year of our Lord", year 1 and that's when 1st century started . yes it is, the first century BC
1st century BC
The year 340 BC is classified as the 4th century BC.
Between the years 599 BCE and 500 BCE