希腊字母表的第四个字母是delta。

The fourth letter of the Greek alphabet is ___delta.

长江三角洲是中国最肥沃的农业区之一。

The Yangtze River ___delta is one of China's most fertile agricultural areas.

两个实验结果的差异很小，几乎可以忽略不计。

The ___delta between the two experimental results was very small, almost negligible.