活化下列何種受體（receptor）能夠最有效地幫助支氣管擴張？
\mathrm{\alpha}_{1} adrenergic receptor
\mathrm{\alpha}_{1}
\mathrm{\beta}_{2} adrenergic receptor
\mathrm{\beta}_{2}
\mathrm{M}_{3} cholinergic receptor
\mathrm{M}_{3}
serotonin 5-\mathrm{HT}_{3} receptor
\mathrm{HT}_{3}
