Are These Numbers in Standard Form?

Question 1:

61× 10 6
No

Question 2:

7.43× 10 4
Yes

Question 3:

12.1× 10 5
No

Question 4:

44.6× 10 3
No

Question 5:

3.74× 10 3
Yes

Question 6:

7.2× 10 6
Yes