蒸汽机的发明是现代工业发展的先驱。

The invention of the steam engine was a ___precursor to modern industry.

传真机是现代电子邮件的先驱。

The fax machine was a ___precursor to modern email.

维生素D是钙吸收的重要前体物质。

Vitamin D is an important ___precursor for calcium absorption.