Standard Form by Boxed Rockets

Are These Numbers in Standard Form?

79 \times 10^{-4}

No

9.06 \times 10^{6}

Yes

0.231 \times 10^{-5}

No

91.1 \times 10^{5}

No

5.81 \times 10^{3}

Yes

0.726 \times 10^{-6}

No