[Modify] Answer Correction And Add Some Explanation

practice-exam/practice-exam-19.md

@@ -244,7 +244,7 @@ If this practice exam has been helpful to you please share it with others and re
 17. Which AWS tool is used to compare the cost of running an application on-premises to running the application in the AWS Cloud?
     - A. AWS Trusted Advisor
     - B. AWS Simple Monthly Calculator
-    - C. AWS Total Cost of Ownership (TCO) Calculator
+    - C. AWS Pricing Calculator
     - D. Cost Explorer
 
     <details markdown=1><summary markdown="span">Answer</summary>

practice-exam/practice-exam-20.md

@@ -147,7 +147,7 @@ If this practice exam has been helpful to you please share it with others and re
 
     <details markdown=1><summary markdown="span">Answer</summary>
 
-    Correct Answer: DE
+    Correct Answer: Ac
 
     </details>
 
@@ -405,7 +405,7 @@ If this practice exam has been helpful to you please share it with others and re
 
     <details markdown=1><summary markdown="span">Answer</summary>
 
-    Correct Answer: CD
+    Correct Answer: BC
 
     </details>
 

practice-exam/practice-exam-5.md

@@ -45,6 +45,10 @@ If this practice exam has been helpful to you please share it with others and re
 
     <details markdown=1><summary markdown='span'>Answer</summary>
       Correct answer: C
+
+      Explanation:
+      - Pricing is per instance-hour consumed for each instance, from the time an instance is launched until it is terminated or stopped.
+      - Each partial instance-hour consumed will be billed per-second for Linux, Windows, Windows with SQL Enterprise, Windows with SQL Standard, and Windows with SQL Web Instances, and as a full hour for all other instance types.
     </details>
 
 5. What is the AWS Support feature that allows customers to manage support cases programmatically?