The expression "a != b" returns true if "a" is not equal to "b." It checks if the values of "a" and "b" are different. The "!=" operator is used for inequality comparisons. The other options represent different comparison operations.

In a 2D array in Java, arr[i] refers to the ith row of the array. This is because a 2D array is essentially an array of arrays, where each element arr[i] is itself an array representing a row. Accessing arr[i] gives you the entire row at index i.

The modular CSS methodology that uses the Block, Element, Modifier structure is BEM, which stands for Block, Element, Modifier. BEM is a naming convention that helps create maintainable and predictable CSS code by organizing styles into blocks, elements, and modifiers.

Excessively long and specific selectors in compiled CSS are often caused by overusing nesting in preprocessors like SASS or LESS. While nesting can improve code organization, it can also lead to overly specific selectors that increase file size and reduce maintainability.

The primary purpose of functions in SASS is to calculate mathematical expressions. SASS functions can perform various mathematical operations, making it easy to compute values for CSS properties dynamically. They are valuable for maintaining consistency and making stylesheets more maintainable.

When you set the child element's font-size to 1.5em, it multiplies the parent element's font-size. In this case, 1.5em x 20px = 30px. So, the computed font-size of the child element will be 30px.

The CSS property that defines the number of times an animation should run is animation-iteration-count. By specifying a value, such as an integer or "infinite," you can control how many times the animation repeats. This property is essential for creating looping animations with a specific number of iterations or an indefinite loop.

To ensure that the distance between lines of text is 1.5 times the size of the text, you can use the line-height property in CSS. You can set it to 150%, which is equivalent to 1.5 times the font size. Alternatively, you can use line-height with an em value, like 1.5em, which achieves the same effect.

The backface-visibility property in CSS is particularly useful when creating 3D transformations. When set to hidden, it prevents the back side of an element from being visible, which is essential for 3D transformations where the front and back of an element should not be visible simultaneously.

In Flexbox, the justify-content property is used to define how items are aligned along the main axis. It controls the distribution of space between and around items along the main axis and is commonly used to center items horizontally or vertically.