linear partial differential operator

例句：

1. 1. The lineof the road is straight as an arrow.
   这条路线直得像箭一样。
2. 2. The datais linearly related to the temperature.
   数据与温度呈线性关系。
3. 3. Thegraph shows a linear increasein income over time.
   图表显示收入随时间呈线性增长。