A:nineteen and ninety-nine
B:nineteen ninety-nine
C:one thousand nine hundred ninety-nine
D:nineteen hundred and ninety-nine
生物体结构和功能的基本单位是( )
zú jì
cǎi sè
gē shēng
chǎo fàn
ā yí
yóu xì
____
zì mǔ
yǒng shì
wàng jì
xìng yùn
shǐ jìn
liú shén
“a=-1”是“直线a2x-y+1=0与直线x-ay-2=0互相垂直”的( )