If xxx is a solution to 15x≡9(mod21)15x \equiv 9 \pmod{21}15x≡9(mod21), which of the following is true?
x≡2(mod7)x \equiv 2 \pmod 7x≡2(mod7)
x≡1(mod7)x \equiv 1 \pmod 7x≡1(mod7)
x≡3(mod7)x \equiv 3 \pmod 7x≡3(mod7)
x≡0(mod7)x \equiv 0 \pmod 7x≡0(mod7)