A polymorphic virus is a complex malicious computer program that changes its binary signature and code appearance every time it infects a new system or file, typically using different encryption keys and decryptor routines.

Why Option 2 is correct: Polymorphic viruses are specifically engineered to change their appearance (underlying code and decryption routine) with each iteration while preserving their core functionality, thereby evading signature-based antivirus scanners.
Other options: Spreading across platforms or deleting files are general malware activities, but changing code appearance is the defining characteristic of polymorphism.

Student's t-distribution is a symmetric bell-shaped distribution centered at 0.

For a two-tailed test of significance, the magnitude of the test statistic $|t|$ determines statistical significance and the p-value.
Since $|-5| = |+5| = 5$, both values lie at the exact same distance into the rejection region in opposite tails, yielding identical two-tailed p-values.