Not having worked out the program, ________ leave the office.
A:so he was forbidden to
B:and he didn;t want to
C:his little son couldn;t make him
D:he couldn't free himself to
若a→与b→为非零向量,|a→+b→|=|a→-b→|,则a→与b→的夹角为____
— Thank you.
— _______.