Edit GoogleSheets "SendEmail" script to only send email if criteria are met - email

Question

I've been searching online all morning, and haven't been able to find what I want to do. I found a similar post on here, but couldn't figure out how to adapt it to my needs.
I'm a teacher, and I was thinking about how I could use Google Sheets to automatically email parents when their child's grade falls below a C in my class. I was able to quickly find the script to have Sheets send an email, and was wondering if it's possible to edit that script to have the email sent ONLY if the value in a specific cell is below 70.
Here's the script I found. I edited the subject line to read "Math Grade" (that was easy to figure out), but I'd like to edit it so that the email only gets sent if the value in cell D2 (or D3, D4, D5, etc) is below 70. (Column A has email, Column B has the message, Column C would have the student's name, and Column D would have their numerical grade.) I feel like it should be obvious, but my scripting knowledge is non-existent. Can anybody help?
function sendEmails() {
var sheet = SpreadsheetApp.getActiveSheet();
var startRow = 2; // First row of data to process
var numRows = 2; // Number of rows to process
// Fetch the range of cells A2:B3
var dataRange = sheet.getRange(startRow, 1, numRows, 2)
// Fetch values for each row in the Range.
var data = dataRange.getValues();
for (i in data) {
var row = data[i];
var emailAddress = row[0]; // First column
var message = row[1]; // Second column
var subject = "Math Grade";
MailApp.sendEmail(emailAddress, subject, message);
}
}

Answer 1

The code was only getting 2 columns of data, and it needs to get 4.
getRange(row, column, numRows, numColumns)
I added a line of code that gets how many columns are in the sheet. Then the for loop got an extra line of code to get the grade, and finally, an if conditional check was added.
function sendEmails() {
var sheet = SpreadsheetApp.getActiveSheet();
var startRow = 2; // First row of data to process
var numRows = 2; // Number of rows to process
var numOfColumns = sheet.getLastColumn();
// Fetch the range of cells
var dataRange = sheet.getRange(startRow, 1, numRows, numOfColumns);
// Fetch values for each row in the Range.
var data = dataRange.getValues();
var grade = "";
var i = 0;
for (i=0;i<data.length;i++) {
var row = data[i];
var emailAddress = row[0]; // First column
var message = row[1]; // Second column
grade = row[3];
if (grade < 70) {
var subject = "Math Grade";
MailApp.sendEmail(emailAddress, subject, message);
};
};
};