Under the Americans with Disabilities Act (ADA) a hotel or restaurant can do which of the following?

a. refuse to allow a guest to register or dine who arrives looking unkempt/untidy/messy
b. refuse to serve someone on the grounds of gender, marital status, or disability
c. refuse to serve someone on the grounds of race, color, religion, or nationality
d. refuse to make modifications under the ADA because the required modifications are not readily available