Meaning of 'should be thinking' or 'should be doing'

[Lora]

English Language Tests, Intermediate level

ESL/EFL Test #123 "English tense explanations", question 10

when I should be thinking about the ..........

(a) initial
(b) genesis
(c) first
(d) beginning

English Language Tests, Intermediate level

ESL/EFL Test #123 "English tense explanations", answer 10

when I should be thinking about the beginning.

Correct answer: (d) beginning
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hi
please explain me the meaning of should be thinking or should be doing...
thanks

Lora

[Conchita]