15 A ..... smile is a sign of friendliness in the USA.

Analysis:

The sentence describes a type of smile that signifies friendliness. A 'wide smile' is commonly associated with friendliness and happiness.

Breakdown:

• A long: Incorrect. 'Long smile' is not a common idiom.
• B high: Incorrect. 'High smile' is not a common idiom.
• C wide: Correct. A 'wide smile' is a broad smile, often indicating happiness and friendliness.

Translated Question:

15 ..... улыбка — признак дружелюбия в США.