In a detection experiment, randy says "yes" to 90% of the trials, and perry says "yes" to 70% of the trials. our best conclusion from this study is

Hi!

In a detection experiment in which Randy says "yes" to 90% of the trials, and Perry says "yes" to 70% of the trials. our best conclusion from this study is that response criterion may be different for Randy and Perry.

We can't say that Randy's threshold is higher than Perry's, as he says yes to most trials, and his threshold is lower.

We can't also say that they are equally sensitive because they respond differently to the trials. 

Finally, we can't also say that  Perry is more sensitive than Randy, because he said "yes" to a fewer percentage of trials, meaning that he is less sensitive than Randy.  

The option that remains, and the right one, is that response criterion may be different for Randy and Perry because they react with a different sensitivity to the same stimulus. 

Have a nice day!