The correct answer is Always thinking of only one thing. Let's look at the meaning of the given idiom: One-track mind: used in reference to a person whose thoughts are preoccupied with one subject or interest.

The correct answer is 'at a fast pace'. In the given sentence 'On the double' is an idiom. The meaning of it is as follows: On the double: very quickly and without any delay.

The correct answer is "beyond one's capability to understand something". Key Points. Given Idiom: Over one's head means too complicated to be understood by one. For example, My jokes always seem to go over one's head.