If $straight theta$ is the angle between two vectors $straight a with rightwards arrow on top space and space straight b with rightwards arrow on top$ , then $straight a with rightwards arrow on top comma space straight b with rightwards arrow on top space greater or equal than space 0$ only when

$0 less than straight theta less than straight pi over 2$
$0 less or equal than straight theta less or equal than straight pi over 2$
$0 less than straight theta less than straight pi$
$0 less or equal than straight theta less or equal than straight pi$

Solution

0 less or equal than straight theta less or equal than straight pi over 2

straight a with rightwards arrow on top. space straight b with rightwards arrow on top space greater or equal than space 0

only if

open vertical bar straight a with rightwards arrow on top close vertical bar space open vertical bar straight b with rightwards arrow on top close vertical bar space cos space straight theta space greater or equal than space 0 comma space

i.e. only if

cos space straight theta space greater or equal than space space 0 comma

i.e., only if

0 less or equal than space straight theta space less or equal than space straight pi over 2.

(B) is correct answer.

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