Who first demonstrated the $C_4$ pathway?

Atriplex spongiosa is a

In Hatch and Slack pathway,the primary $CO_2$ acceptor is

$A$: Tropical plants are more efficient in $CO_2$ utilization.
$R$: $C_3$ plants ensure that $RuBisCO$ functions as carboxylase,minimizing oxygenase activity.

The $C_4$ pathway (Hatch and Slack cycle) is found in which of the following plants?

Which of the following is a $C_4$ plant?