In area-amplitude ultrasonic standard test blocks, the flat-bottomed holes in the blocks are:

All the same diameter.

Different in diameter, increasing in 0.4 mm (0.02 in.) increments from the No. '1' block to the No. '8' block.

Largest in the No. '1' block and smallest in the No. '8' block.

Drilled to different depths from the front surface of the test block.

None of the above.