The expense related to the share options granted to the sales employees should be calculated using the fair value of the options, which is $10 per option. Since the options vest if Tucan reaches its annual revenue target of $1,000,000 during any of the four-year service period, it is a performance-based equity-settled share option. According to the "企业会计准则第11号——股份支付（2006）" and the "企业会计准则第9号——职工薪酬（2014）", the expense for such share options should be recognized over the vesting period, which in this case is four years.

However, since Tucan expects to meet the revenue target in 2021, the expense for the entire grant should be recognized in the year the options are granted because the performance condition is deemed to be met. Therefore, the full grant expense should be recognized in 2018.

The expense at the end of 2018 would be the total grant expense, which is calculated as the fair value of the options multiplied by the number of options granted. The calculation is as follows:

1,000 options/employee * 50 employees * $10/fair value per option = 50,000 options * $10/fair value per option = $500,000

However, since the options vest if the revenue target is met during any of the four-year service period and Tucan expects to meet the target, the full grant of the options0 options granted to vest in 2021, the expense08000000 options granted, and all employees are expected to remain with the Company through that date. The expense at the end of the year is $500,000, which should be recognized in 20188.

So, the expense at the end of 2018 would be $500,000.

Therefore, the expense at the end of 2018 is $500,000, which should be recognized in 20188, representing the cost of the options000 options granted.

Hence, the expense at the end of 20188 is $500,000.

The expense at the end of 2018, considering the expected achievement of the revenue target and the assumption that all employees will remain with the company through 2021, is $500,000.

So, the expense at the end of 2018 is $500,000.

The answer is C. $500,000.