Answer: go directly to sleep.
Step-by-step explanation:
Retroactive interference is the phenomena in which presently learned information can disturb the earlier learned information in mind.
According to the mentioned situation, Ricardo should immediately go to sleep .As he is feeling that he cannot study anymore because he has been studying for long duration of time .
If he keeps on studying, retroactive interference can take place which will hinder the already studied knowledge with the current knowledge .By sleeping, his brain can relax and will get time recover the ability to learn more information after getting proper sleep.