8th grade B

Expressing obligation, permission and advice

http://www.ecenglish.com/learnenglish/lessons/modal-verbs-1-permission-prohibition-obligation-no-obligation

http://web2.uvcs.uvic.ca/elc/studyzone/410/grammar/410-modals-of-necessity-prohibition-and-permission1.htm

http://www.perfect-english-grammar.com/modal-verbs-of-obligation-exercise-1.html

http://www.tolearnenglish.com/exercises/exercise-english-2/exercise-english-12439.php