Which species cannot function as a Lewis acid?

A.     ${\text{B}}{{\text{F}}_{\text{3}}}$

B.     ${\text{AlC}}{{\text{l}}_{\text{3}}}$

C.     ${\text{CC}}{{\text{l}}_{\text{4}}}$

D.     ${{\text{H}}^ + }$

C

Whilst 42% correctly identified ${\text{CC}}{{\text{l}}_{\text{4}}}$ the most popular wrong answer (38%) was the ${{\text{H}}^ + }$ ion.