The ________ thread has a ________ diameter and steeper taper, which gives it greater strength than the _______ thread.

• A. Mueller, larger, iron-pipe
• B. National thread has a larger diameter than iron-pipe
• C. Whitworth thread has a larger diameter and steeper taper
• D. American thread has a larger diameter

Answer: (A)

The main idea is that threaded strength depends on the geometry of the threads: a larger major diameter and a steeper taper mean more material engaged in the thread and a deeper bite into the nut or fitting, which strengthens the connection and helps resist leakage and pull-out under pressure.

Mueller thread is described as having a larger diameter and a steeper taper than the iron-pipe thread, so it provides greater engagement and strength. That combination—larger diameter plus a steeper taper—gives a more robust threaded connection in practice, which is why this option is the correct one.

The other choices don’t fit both parts of the description. They either mention only diameter or only taper, or compare threads in a way that doesn’t pair Mueller with a weaker iron-pipe thread in the same way.