Shruthi Singerr, a citizen and resident of country M$,$ is a musician that puts on $5$ shows on her United States tour. To minimize overhead, Shruthi manages the ticket booth before her concerts begin, and the concession cart during intermission, selling tickets and beverages, respectively. After the shows, Singerr sells T$-$shirts.

a$.$Is Singerr engaged in a U$.$S$.$ trade or business? Explain.

b$.$If Singerr is engaged in a U$.$S$.$ trade or business, what income is subject to U$.$S$.$ tax? Is any income subject to withholding tax?

c$.$ When does Singerr pay any U$.$S$.$ tax? What form does she need to file?