Your network contains two Active Directory forests. One forest contains two domains named contoso.com and na.contoso.com. The other forest contains a domain named nwtraders.com. A forest trust is configured between the two forests. You have a user named User1 in the na.contoso.com domain. User1 reports that he fails to log on to a computer in the nwtraders.com domain by using the user name NA\User1. Other users from na.contoso.com report that they can log on to the computers in the nwtraders.com domain. You need to ensure that User1 can log on to the computer in the nwtraders.com domain. What should you do（）

A. Enable selective authentication over the forest trust.
B. Create an external one-way trust from na.contoso.com to nwtraders.com.
C. Instruct User1 to log on to the computer by using his user principal name （UPN）.
D. Instruct User1 to log on to the computer by using the user name nwtraders\User1.