You use a 4-digit passkey to unlock your smartphone. However, you are worried that your phone's security may be compromised since your younger sibling carefully watched you unlock your phone.

a. How many combinations will your sibling need to try if they determined the first digit of your passkey?

b. How many combinations will your sibling need to try if they determined the last digit of your passkey?

c. How many combinations will your sibling need to try if they determined the first two digits of your passkey?

d. What can be done to increase your phone's security?