Least effective were chlortetracycline, oxytetracycline, and tetracycline.是什么意思？

Least effective were chlortetracycline, oxytetracycline, and tetracycline.

效力最差的是金霉素 、 土霉素和四环素.

least

pron. 最小的，最少的，程度最轻的；adv. 最少，最小；

例句
Balsamic vinegar is aged in wood for at least 10 years and the flavour is wonderful...
意大利香醋在木桶中就经历了至少10年的酝酿期，所以口味极佳。

effective

adj. 有效的，起作用的，实际的，实在的，给人深刻印象；n. [军]现役兵额，有生力量，精兵；

例句
Light colours provide an effective backdrop for pictures or a mirror.
浅色是挂放图画或镜子很有效果的衬托。

were

be的过去时复数和第二人称单数形式，有时代替 was,用于条件从句、动词 wish 之后等；

例句
Shops, houses, and vehicles were set ablaze.
店铺、房子和车辆全都陷于火海。

chlortetracycline

n. 氯四环素；

and

conj. 和，与，而且，于是，然后，因此；

例句
I spend a lot on expensive jewelry and clothing...
我花很多钱购买昂贵的首饰和衣服。