在填写表格之前，用户名那一栏显示的是占位文本。

Before filling out the form, the username field shows a ___placeholder text.

在找到正式老师之前，学校安排了一位代课老师。

The school assigned a ___placeholder teacher until they find a permanent one.

在代数公式中，字母y通常用作未知数的占位符。

In algebraic formulas, the letter y is often used as a ___placeholder for the unknown value.