I want to select in Mat Table with selection Based on dropdown value in angular

I have created a Table with selection using angular material. Now I want to check some particular rows in onchange method of a dropdown. I did so far:

1. Created the table
2. Load value in database
3. Created dropdown and onChange method
4. Created a save button that gives me the selected value and my dropdown value.

Now I am unable to check the row for the value based on onChange of the dropdown. My dataSource:

const ELEMENT_DATA: PeriodicElement[] = [
  { position: 1, name: 'Hydrogen', weight: 1.0079, symbol: 'H' },
  { position: 2, name: 'Helium', weight: 4.0026, symbol: 'He' },
  { position: 3, name: 'Lithium', weight: 6.941, symbol: 'Li' },
  { position: 4, name: 'Beryllium', weight: 9.0122, symbol: 'Be' },
  
];

In onChange of my dropdown let say I want to check only this two-row

 { position: 1, name: 'Hydrogen', weight: 1.0079, symbol: 'H' },
  { position: 2, name: 'Helium', weight: 4.0026, symbol: 'He' },

Noted:I should be able to check or uncheck after loading value and get a new checked value in my save button Here is my code in stackblitz