To customise options press the '...' button on the top right.
You can either set custom weights (make sure to press the use weights toggle on the top right) for the options or have them be equally weighted. Weights must add up to 100 but they can be decimals.
To set weights for each option type them in like: <Option1>:<Weight1>, <Option2>:<Weight2>, etc....
e.g. a:50, b:50 OR a:2.5, b:97.5
If not using weights make sure to type using this format: <Option1>,<Option2>,.....
e.g. a, b, c