为什么是4年而不是用平均的5年

问题如下：

On January 1, 2011, the company granted 50,000 options on its common shares to its top managers. The options have the following features:

The exercise price for all company stock options is the stock price on the date of the grant.

Options cannot be exercised for 4 years and expire in 10 years from the grant date.

Based on prior experience, it is estimated that options are exercised, on average, in five years.

The related compensation expense is reported in selling and administrative expenses.

Exhibit 3 contains stock information and estimated option prices during 2011; option prices are estimated using the Black–Scholes model using assumptions contained in the Notes to the Financial Statements.

The compensation expense for 2011 arising from the executive stock options granted in 2011 is closest to:

选项：

A.

$43,800.

B.

$54,750.

C.

$76,625.

解释：