Question

a the growth is approximately linear, since the percentage share of low - cost seats increases by roughly 13% every year.
b the growth is approximately linear, since the percentage share of low - cost seats increases by roughly 13% every 2 years.
c the growth is approximately exponential, since the percentage share of low - cost seats grows at an average rate of 1.3 times each year.
d the growth is approximately exponential, since the percentage share of low - cost seats grows at an average rate of 1.3 times every 2 years.

Explanation:

To determine the correct option, we analyze linear and exponential growth:

• Linear growth has a constant absolute increase over time (e.g., +13% per year or +13% every 2 years, but the rate per unit time must be constant).
• Exponential growth has a constant relative increase (e.g., multiplying by 1.3 per year or every 2 years, meaning a percentage increase relative to the current value).

Analyzing each option:

• Option A: Claims 13% growth per year. For linear growth, the absolute increase per year should be constant. However, if the base (total seats or low - cost seats) changes, a constant percentage increase (13% of a growing base) is exponential, not linear. So A is incorrect.
• Option B: Claims 13% growth every 2 years. To check linearity, we calculate the annual absolute increase: $\frac{13\%}{2}=6.5\%$ per year (constant absolute increase). This fits linear growth: the percentage share increases by a constant absolute amount (6.5% per year, or 13% every 2 years).
• Option C: Claims "1.3 times each year" (exponential growth). But "1.3 times" implies a 30% annual growth rate. However, the problem does not indicate a 30% annual relative growth (the description of "13%" in other options suggests a 13% absolute or relative increase, not 30%). So C is inconsistent.
• Option D: Claims "1.3 times every 2 years" (exponential growth). But the problem’s context (mention of "13%") does not align with a 1.3 - fold (30%) growth every 2 years. Also, the key for linear vs. exponential is misapplied here, as the "13%" in other options points to absolute growth, not 1.3 - fold relative growth.

Answer:

B. The growth is approximately linear, since the percentage share of low - cost seats increases by roughly 13% every 2 years.