An officer who smells marijuana as he drives by a car with an open window:
a.
a. cannot compel the driver to stop the car so he can investigate without a warrant.
b.
b. is limited to noting the license plate number of the vehicle and initiating a stake-out of the owner's home in order to gather additional evidence.
c.
c. is authorized to stop the car to investigate without a warrant or probable cause because it is a plain search and outside the scope of the fourth amendment.
d.
d. is