What is a symbiotic relationship example similar to barnacles on a humpback whale

The relationship between barnacles and whales is a Commensalism relationship where only the barnacles benefit from the relationship but the whales are not harmed. Another example of a Commensalism relationship would be the Remora fish which is a fish that attaches itself to bigger fish and sharks to get a ride to other places, and does not harm the bigger fish nor does it benefit them.

i think a) betsy can donate only blood to mark is correct because, mark and betsy have the same blood type. whereas nobody else has b+ blood so betsy can't donate to them.