How to create a question where participants have to choose between two answers? but I don't want them to be able to answer both

Hi,

I need to set up a question where I ask participants for how its been since they did some training. 

They can answer either in months (if recent) or in years. 

So the question is...how long has it been since you last completed supervision skills training….either provide your answer in months or years. 

answer 1 (months): drop down options for number of months

answer 2 (years): drop down options for number of years.

please could you recommend the best way to set this up?

I don’t want them to be able to complete both answers. 

Many thanks,

Catrin