Which of the following for loop statements is not valid?

Select one:

a$.$ for int $i=99$;$13=0$;$\frac{1}{9}$

b$.$ for $($int $\{$:$i=2$;$i20*i=2^{**}i)$

c$.$ for $($ int $i=20$;$i2$;$-i)$

d$.$ for $($int $i=7$;$i77i+=7$