Question

a and bbb are in direct variation.

Which of these equations could represent the relationship between aaa and bbb?

19⋅a=b\frac{1}{9} \cdot a = b​9​​1​​⋅a=b
a⋅b=9 a \cdot b = 9 a⋅b=9
a+b=9 a + b = 9 a+b=9
9⋅a=1b 9 \cdot a = \frac{1}{b} 9⋅a=​b​​1​​
19⋅1a=b \frac{1}{9} \cdot \frac{1}{a} = b ​9​​1​​⋅​a​​1​​=b