$1\mathrm{.}3$ Overloading. Which of the following expressions uses correct notation? If the notation is correct, is it also unambiguous? Assume that $a$ is a $10-$vector and $b$ is a $20-$vector.

$($a$)b=(0,a).$

$($b$)a=(0,b).$

$($c$)b=(0,a,0).$

$($d$)a=0=b.$