These missions may not always be the most dangerous but only ANBU members or the Kage participate in them. It is very unlikely anyone of a lower rank would be part of these missions.
This type of mission is the most dangerous. Ninjas ranked Jounin or higher are usually assigned to these missions. Very often, if things are very bad you will see a Kage participating. Generally you will not find a ninja of lower rank than Jounin in these types of missions unless it is absolutely necessary. Rank A missions are extremely dangerous and usually there is a death or severe injury as a result of it.
Missions B ranked are not as tough as A ranked missions. The difficulty is still pretty high. Most B ranked missions involve retrieving important documents or protecting someone and taking them to a certain location. Mostly Jounin or Chuunin ranked ninjas perform these types of missions.
These types of missions are generally performed by Chuunins. At times you will even see skilled Genins performing missions C ranked. Most C ranked missions involve protecting an elderly or sending/retrieving a document.
D ranked missions are the lowest ranked. These missions are almost all performed by Genins. These missions usually are tedious work or basically useless work. Some missions include: saving an animal, taking out the garbage, walking an animal, or other pointless and boring work. Genins who want to be Chuunins must complete 8 of these missions.
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