One contrast presented in the first paragraph is between A) talkative and uncommunicative children
B) graceful and clumsy movement
C) formal and informal learning
D) substantive and superficial mistakes
E) mandatory and optional instruction

modal adverbs are used to modify specific verbs that consist of a linking verb (verb of being) and sometimes another verb. examples of modal verbs include can, could, may, might, must, shall, should, will and would.

modal auxiliary. an auxiliary verb characteristically used with other verbs to express mood, aspect, or tense. in english, the most common modal auxiliaries are can, could, may, might, must, ought, shall, should, will, and would. also called modal .

glitch or non-saved account.

explanation:

i'm not personally a mobile user but one reason could be that it is either a glitch where it doesn't automatically log you in, or you have it so cant save your log-in information. i suggest you use the mobile app instead of using your web browser!