Which of the following is not a valid Huffman tree? Which of the following is not a valid Huffman tree?

$($b$)$

a$,$ b$,$ c$,$ d$,$ e: $166$

$($b$)$

a$,$ b$,$ c$,$ d$,$ e: $166($c$)a,b,c,d$:$100$

$($d$)$

$a,b,c,d,e$: $81$

tree $($a$)$

tree $($b$)$

tree $($c$)$

tree $(d)$