In a certain code language. ‘Keep your eyes open’ is written as ‘nx co kh wd’ ; ‘eyes on school bus’ is written as ‘vy so rs te’ ; ‘keep bag in bus’ is written as ‘rs ef bn kh’ ; ‘open your school bag’ is written as ‘wd ef nnx vy’ (All codes are two-letter codes only) In the given code language, what does the code ‘nx’ stand for