Dan Kay is a citizen and resident of Brazil, which does not have a tax treaty with the United States. He conducts a three$-$concert tour in the United States. A U$.$S$.$ based promoter rents the venues and sells the tickets to Dan concert from the ticket booth before the concert begins. The promoter then pays Dan for his performances. A multi$-$talented individual, during intermission, Dan sells beverages from the concession stand. After the concert, Dan sells T$-$shirts at the concession stand.

$($a$)$ Is Dan Kay engaged in a U$.$S$.$ trade or business? If so$,$ what type of income$($s$)$ does Dan generate? Please explain.

$($b$)$ If Dan Kay is engaged in a U$.$S$.$ trade or business, what income$($s$)$ is subject to U$.$S$.$ tax? When does Dan Kay pay U$.$S$.$ taxes on the income$($s$).$